Unmet dependency with --backports=true on Debian stable

[Thierry <deblist@sagedebianlive.metelu.net>]

Hi,

a regression appeared recently in live-build in stretch: if i enable
"--backports=true" in lb_config, i can not build the live anymore, since
i got the following error:

> The following packages have unmet dependencies:
>   network-manager-config-connectivity-debian : Depends: network-manager (>= 1.14.4-2~bpo9+1) but 1.6.2-3 is to be installed

Surprisingly, it seems that nothing depends on
network-manager-config-connectivity-debian as i got (from my own
computer running stretch as well and with backports enabled):

> $ apt-cache rdepends network-manager-config-connectivity-debian
> network-manager-config-connectivity-debian
> Reverse Depends:

So, my questions are: what triggers the installation of
network-manager-config-connectivity-debian in live-build, and how to fix
the issue ?

Best regards,
Thierry