Paint masks disappeared on black masks

Hello,
I often use paint masks on black masks to designate areas where I want to assign my materials.
Yesterday, I worked on a file with a lot of materials, using black masks and paint masks to assign them.
This morning, when I reopened my file, some paint masks had visually disappeared from the black masks, yet the areas in question were still correctly painted.
The black masks appear visually empty, so they shouldn’t be painting any areas.
When I delete the black masks, the material is applied normally to the entire object, which shows that something specific was being applied.

Have you encountered this bug?
Thank you in advance,
Best regards.