Missing masks since recent updates

Since one of the recent updates, I find my older images have lost their masks, and C1 replaces them with an empty mask. It keeps the adjustments, but not where they have been applied. Unfortunately each time I open the catalog, more images seem to be affected, with newer and newer ones suffering the problem each time. This is going to result in an incredible amount of rework, as restoring from backups does not seem to find them again.. Now been with C1 support for weeks, sent logs, backups etc, and having to chase them again.

I see this problem reported on the forum from a few years back but has gone quiet recently. Anyone else suffering the problem recently, or any advice on how to stop the problem spreading to other images and how to get the masks back?