[UPDATE] I've furtherly narrowed down the process (or just one of many?) to recreate the issue: .: I trace a series of masked areas over a previously existing polygroup :. I open the Polygroups menu and click on "Group Masked" .: I Ctrl+click&drag on the canvas to clear masks :. I save the project .: I rotate the camera...…
[UPDATE] Ok, I've finally managed to witness the exact moment when the polygroup disappears, here the conditions: .: nails' heads polygrouped :. nail's heads masked/inverted (all the rest is masked, not the nails) .: automatic quick OR manual save runs At this point the nails polygroup disappears, absorbed by the…