System Information
Operating system: Linux-5.4.0-42-generic-x86_64-with-debian-bullseye-sid 64 Bits
Graphics card: Quadro GP100/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 440.100

Blender Version
Broken: version: 2.90.0 Beta, branch: master, commit date: 2020-08-02 18:54, hash: rB0264f53e30c7

Short description of error
When sculpting in multires, the switching between different levels of subdivision is not included in the undo stack.
Because of this, when sculpting changes in the model, then switching between levels and sculpting more and afterwards using undo, it will not reliably undo the strokes.
In some cases it won't undo at all unless it exits sculpt mode or reaches some point in the undo stack where the level gets changes another time.

Exact steps for others to reproduce the error

• Add an object with a multiresolution modifier and subdivide it a few times
• Sculpt changes on the highest level, then on a lower level and then again on a higher level
• Undo all those strokes until the mesh breaks or you exit sculpt mode and all is reverted at the same time