xapps-common is tagged as architecture:all, but the generated xapp-sn-watcher.desktop included in it depends on the architecture it was built on.
By accident, we rebuilt the arch:all packages from xapp on i386, while our main architecture is amd64.
The result is an xapp-sn-watcher.desktop file that isn't functional, as it leads to:
systemd-xdg-autostart-generator: Exec binary '/usr/lib/i386-linux-gnu/xapps/sn-watcher/xapp-sn-watcher' does not exist: No such file or directory
Similarly, if someone would install Debian bullseye with a main architecture i386 (not many would do, but there are certainly some) or an ARM architecture, this would also fail (because there, it refers to the x86_64 library directory). Thus I'm setting the severity to Serious here - It is broken for any architecture other than amd64 in Debian.
There are probably two possible fixes (maybe more):
1) Move the autostart file into the respective libxapp1 packages (with the binary)
2) Use the alternatives mechanism to let one binary package provide the executable at a common location.