Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: AMD Radeon RX 5700 XT ATI Technologies Inc. 4.5.13587 Core Profile Context 20.4.2 26.20.15029.27017
Broken: after face sets implementation
Tested in 2.91.0 Alpha, branch: master, commit date: 2020-08-28 00:49, hash: rB5ee60c981573
Short description of error
When sculpting with dyntopo if you undo sometimes part of the mesh simply dissappear.
Exact steps for others to reproduce the error
-open the file below.
-change the size of the dyntopo detail to something small like 2 or 3px
-get close to the object and use the brush once
If you make it in the right way you can get it everytime
You can make the geometry re-appear by leaving sculpt mode. Once you go back, note that the geometry that was hidden is now a face set.
I think it may be related to https://developer.blender.org/T74637