buildstream hangs in operations with --except

I was wondering what the hangs are, when buildstream is clearly doing nothing, now I find it is 100% reproducible.

Any command where --except is specified and the scheduler is run, it will result in a hang.

For example with the GNOME project:

bst track --deps all --except base.bst core/meta-gnome-core.bst

This will consistently result in a hang.