[Bug]: FLATPAK_SPAWN_FLAGS_WATCH_BUS does not work in some cases

[sophie-h]

Checklist

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a bug that matches the one I want to file, without success.
• If this is an issue with a particular app, I have tried filing it in the appropriate issue tracker for the app (e.g. under https://github.com/flathub/) and determined that it is an issue with Flatpak itself.
• This issue is not a report of a security vulnerability (see here if you need to report a security issue).

Flatpak version

1.14.4

What Linux distribution are you using?

Debian

Linux distribution version

12

What architecture are you using?

x86_64

How to reproduce

1. flatpak run --command=sh org.gnome.Loupe
2. Start process via loupe& in sandbox
3. Use Loupe to view images which uses flatpak spawn --watch-bus to spawn image loaders
4. SIGKILL process id of loupe
5. Exit the flatpak run
6. Check flatpak ps. The loupe process is still listed, but the pid does not exist. Probably due to the loupe process still being listed, the processes spawned with --watch-bus are not killed.

Expected Behavior

Whenever the loupe process does not longer exist, the --watch-bus processes it spawned should be terminated.no longer exists

Actual Behavior

The --watch-bus processes keep existing until system restart.

It looks like it's possible to also trigger the bug in other ways than SIGKILL. But that is not sure and unclear how.

Additional Information

No response