Broken: At least 2.83, 2.90, 2.91, current master.
Worked: ???

When starting from File -> New -> Sculpting, draw two strokes, undo twice to go back to starting sphere, redo once (nothing happens), redo again (both first strokes re-appear simultaneously).

Note that this is not reproducible when using default startup, and then switching to Sculpt mode.

I believe this is because in the first case, initial undo step (memfile one) has active object already in Sculpt mode, and the first Sculpt undo step is actually stored for the first stroke.
Conversely, when switching to Sculpt mode from Object mode, that switch itself is stored as a Sculpt undo step, and the first stroke is the second Sculpt undo step.

Further investigation is needed though.