Why apt suddenly lists packages from "stable-backports" as upgrade candidates for packages from "stable"?

["Alexander V. Makartsev" <avbetev@gmail.com>]

    Hi all.
    
    Today apt is acting strange.
    It lists packages from "stable-backports" as upgrade candidates for
    packages from "stable".
    I don't use apt pinning for "stable-backports" or any custom apt
    preference that would force backports installation.
    I do use external repositories, but all of them are reputable
    sources (Steam\Valve, WineHQ, Mozilla, Microsoft, etc.) 
    and "apt policy" output shows those candidates are not from these
    external repos.
    This can't be right.
    A wild guess, is this some new behavior that stems from new apt
    sources format?
    
    
    $ cat /etc/apt/sources.list.d/debian.sources
    Types: deb
    URIs: https://deb.debian.org/debian
    Suites: trixie trixie-updates trixie-proposed-updates
    trixie-backports
    Components: main contrib non-free non-free-firmware
    Signed-By: /usr/share/keyrings/debian-archive-keyring.gpg
    
    Types: deb
    URIs: https://security.debian.org/debian-security
    Suites: trixie-security
    Components: main contrib non-free non-free-firmware
    Signed-By: /usr/share/keyrings/debian-archive-keyring.gpg
    
    
    $ apt list --upgradable
    chromium-common/proposed-updates,stable-security
    142.0.7444.175-1~deb13u1 amd64 [upgradable from:
    142.0.7444.134-1~deb13u1]
    chromium-sandbox/proposed-updates,stable-security
    142.0.7444.175-1~deb13u1 amd64 [upgradable from:
    142.0.7444.134-1~deb13u1]
    chromium/proposed-updates,stable-security 142.0.7444.175-1~deb13u1
    amd64 [upgradable from: 142.0.7444.134-1~deb13u1]
    code/stable 1.106.3-1764110892 amd64 [upgradable from:
    1.103.0-1754517494]
    curl/stable-backports 8.16.0-4~bpo13+1 amd64 [upgradable from:
    8.14.1-2+deb13u1]
firefox-esr-l10n-ru/proposed-updates,proposed-updates,stable-security,stable-security
    140.5.0esr-1~deb13u1 all [upgradable from: 140.3.0esr-1~deb13u1]
    firefox-esr/proposed-updates,stable-security 140.5.0esr-1~deb13u1
    amd64 [upgradable from: 140.3.0esr-1~deb13u1]
    firefox/mozilla 145.0.2~build1 amd64 [upgradable from:
    144.0.2~build1]
krita-data/proposed-updates,proposed-updates,stable-security,stable-security
    1:5.2.9+dfsg-1+deb13u1 all [upgradable from: 1:5.2.9+dfsg-1]
    krita/proposed-updates,stable-security 1:5.2.9+dfsg-1+deb13u1 amd64
    [upgradable from: 1:5.2.9+dfsg-1+b1]
    libbrlapi0.8/stable-backports 6.8-4~bpo13+1 amd64 [upgradable from:
    6.7-3.1+deb13u2]
    libcpupower1/stable-backports 6.17.8-1~bpo13+1 amd64 [upgradable
    from: 6.12.57-1]
    libcurl3t64-gnutls/stable-backports 8.16.0-4~bpo13+1 amd64
    [upgradable from: 8.14.1-2+deb13u1]
    libcurl3t64-gnutls/stable-backports 8.16.0-4~bpo13+1 i386
    [upgradable from: 8.14.1-2+deb13u1]
    libcurl4t64/stable-backports 8.16.0-4~bpo13+1 amd64 [upgradable
    from: 8.14.1-2+deb13u1]
    libcurl4t64/stable-backports 8.16.0-4~bpo13+1 i386 [upgradable from:
    8.14.1-2+deb13u1]
    libldb2/stable-backports 2:2.11.0+samba4.23.3+dfsg-1~bpo13+1 amd64
    [upgradable from: 2:2.11.0+samba4.22.6+dfsg-0+deb13u1]
    libnss-winbind/stable-backports 2:4.23.3+dfsg-1~bpo13+1 amd64
    [upgradable from: 2:4.22.6+dfsg-0+deb13u1]
    libpam-winbind/stable-backports 2:4.23.3+dfsg-1~bpo13+1 amd64
    [upgradable from: 2:4.22.6+dfsg-0+deb13u1]
    libsmbclient0/stable-backports 2:4.23.3+dfsg-1~bpo13+1 amd64
    [upgradable from: 2:4.22.6+dfsg-0+deb13u1]
    libtalloc2/stable-backports 2:2.4.3+samba4.23.3+dfsg-1~bpo13+1 amd64
    [upgradable from: 2:2.4.3+samba4.22.6+dfsg-0+deb13u1]
    libtdb1/stable-backports 2:1.4.14+samba4.23.3+dfsg-1~bpo13+1 amd64
    [upgradable from: 2:1.4.13+samba4.22.6+dfsg-0+deb13u1]
    libtevent0t64/stable-backports 2:0.17.1+samba4.23.3+dfsg-1~bpo13+1
    amd64 [upgradable from: 2:0.16.2+samba4.22.6+dfsg-0+deb13u1]
    libwbclient0/stable-backports 2:4.23.3+dfsg-1~bpo13+1 amd64
    [upgradable from: 2:4.22.6+dfsg-0+deb13u1]
    linux-cpupower/stable-backports 6.17.8-1~bpo13+1 amd64 [upgradable
    from: 6.12.57-1]
    linux-headers-amd64/stable-backports 6.17.8-1~bpo13+1 amd64
    [upgradable from: 6.12.57-1]
    linux-image-amd64/stable-backports 6.17.8-1~bpo13+1 amd64
    [upgradable from: 6.12.57-1]
    linux-libc-dev/stable-backports,stable-backports 6.17.8-1~bpo13+1
    all [upgradable from: 6.12.57-1]
    python3-brlapi/stable-backports 6.8-4~bpo13+1 amd64 [upgradable
    from: 6.7-3.1+deb13u2]
    python3-ldb/stable-backports 2:2.11.0+samba4.23.3+dfsg-1~bpo13+1
    amd64 [upgradable from: 2:2.11.0+samba4.22.6+dfsg-0+deb13u1]
    python3-samba/stable-backports 2:4.23.3+dfsg-1~bpo13+1 amd64
    [upgradable from: 2:4.22.6+dfsg-0+deb13u1]
    python3-talloc/stable-backports 2:2.4.3+samba4.23.3+dfsg-1~bpo13+1
    amd64 [upgradable from: 2:2.4.3+samba4.22.6+dfsg-0+deb13u1]
    python3-tdb/stable-backports 2:1.4.14+samba4.23.3+dfsg-1~bpo13+1
    amd64 [upgradable from: 2:1.4.13+samba4.22.6+dfsg-0+deb13u1]
    samba-ad-dc/stable-backports 2:4.23.3+dfsg-1~bpo13+1 amd64
    [upgradable from: 2:4.22.6+dfsg-0+deb13u1]
    samba-ad-provision/stable-backports,stable-backports
    2:4.23.3+dfsg-1~bpo13+1 all [upgradable from:
    2:4.22.6+dfsg-0+deb13u1]
    samba-common-bin/stable-backports 2:4.23.3+dfsg-1~bpo13+1 amd64
    [upgradable from: 2:4.22.6+dfsg-0+deb13u1]
    samba-common/stable-backports,stable-backports
    2:4.23.3+dfsg-1~bpo13+1 all [upgradable from:
    2:4.22.6+dfsg-0+deb13u1]
    samba-dsdb-modules/stable-backports 2:4.23.3+dfsg-1~bpo13+1 amd64
    [upgradable from: 2:4.22.6+dfsg-0+deb13u1]
    samba-libs/stable-backports 2:4.23.3+dfsg-1~bpo13+1 amd64
    [upgradable from: 2:4.22.6+dfsg-0+deb13u1]
    samba/stable-backports 2:4.23.3+dfsg-1~bpo13+1 amd64 [upgradable
    from: 2:4.22.6+dfsg-0+deb13u1]
    smbclient/stable-backports 2:4.23.3+dfsg-1~bpo13+1 amd64 [upgradable
    from: 2:4.22.6+dfsg-0+deb13u1]
    tdb-tools/stable-backports 2:1.4.14+samba4.23.3+dfsg-1~bpo13+1 amd64
    [upgradable from: 2:1.4.13+samba4.22.6+dfsg-0+deb13u1]
thunderbird-l10n-ru/proposed-updates,proposed-updates,stable-security,stable-security
    1:140.5.0esr-1~deb13u1 all [upgradable from:
    1:128.14.0esr-1~deb13u1]
    thunderbird/proposed-updates,stable-security 1:140.5.0esr-1~deb13u1
    amd64 [upgradable from: 1:128.14.0esr-1~deb13u1]
    winbind/stable-backports 2:4.23.3+dfsg-1~bpo13+1 amd64 [upgradable
    from: 2:4.22.6+dfsg-0+deb13u1]
    wine-devel-amd64/stable 10.20~trixie-1 amd64 [upgradable from:
    10.15~trixie-1]
    wine-devel-i386/stable 10.20~trixie-1 i386 [upgradable from:
    10.15~trixie-1]
    wine-devel/stable 10.20~trixie-1 amd64 [upgradable from:
    10.15~trixie-1]
    winehq-devel/stable 10.20~trixie-1 amd64 [upgradable from:
    10.15~trixie-1]
    xbrlapi/stable-backports 6.8-4~bpo13+1 amd64 [upgradable from:
    6.7-3.1+deb13u2]
    yt-dlp/stable-backports,stable-backports 2025.11.12-1~bpo13+1 all
    [upgradable from: 2025.10.22-1~bpo13+1]
    
    
    $ apt policy curl
    curl:
      Installed: 8.14.1-2+deb13u1
      Candidate: 8.16.0-4~bpo13+1
      Version table:
         8.16.0-4~bpo13+1 100
            100 https://deb.debian.org/debian trixie-backports/main
    amd64 Packages
     *** 8.14.1-2+deb13u1 100
            100 /var/lib/dpkg/status
         8.14.1-2 500
            500 https://deb.debian.org/debian trixie/main amd64 Packages
    
    $ apt policy linux-headers-amd64
    linux-headers-amd64:
      Installed: 6.12.57-1
      Candidate: 6.17.8-1~bpo13+1
      Version table:
         6.17.8-1~bpo13+1 100
            100 https://deb.debian.org/debian trixie-backports/main
    amd64 Packages
     *** 6.12.57-1 100
            100 /var/lib/dpkg/status
         6.12.48-1 500
            500 https://security.debian.org/debian-security
    trixie-security/main amd64 Packages
         6.12.43-1 500
            500 https://deb.debian.org/debian trixie/main amd64 Packages
    
    $ apt policy samba
    samba:
      Installed: 2:4.22.6+dfsg-0+deb13u1
      Candidate: 2:4.23.3+dfsg-1~bpo13+1
      Version table:
         2:4.23.3+dfsg-1~bpo13+1 100
            100 https://deb.debian.org/debian trixie-backports/main
    amd64 Packages
     *** 2:4.22.6+dfsg-0+deb13u1 100
            100 /var/lib/dpkg/status
         2:4.22.4+dfsg-1~deb13u1 500
            500 https://deb.debian.org/debian trixie/main amd64 Packages

--

 With kindest regards, Alexander.

 Debian - The universal operating system
 https://www.debian.org