The changelog being below, but specifically fix #7271 is the fix for this issue. Updated pve-container to 6.1.1, stopped a container with a bind mount and started it again. No issues so far.
pve-container (6.1.1) trixie; urgency=medium
*...
It also seems to be fixed in the newest version so I would try to do another update. Staying in an old version too long can also lead to problems if some other component gets updated which will need a newer version of the pinned package
If you check the bugtracker, there is a patch available as of today to be code reviewed: https://bugzilla.proxmox.com/show_bug.cgi?id=7271#c6
Once reviewed and applied, it will be part of one of the next versions of the pve-container package...
It looks like you run into the bug reported here: https://bugzilla.proxmox.com/show_bug.cgi?id=7271
Feel free to chime in there!
To temporarily downgrade pve-container you can run apt install pve-container=6.0.18