Blender Version
Broken: 2.80, a1ae04d15a9f

Short description of error
If your object has Mask modifier, entering sculpt mode on a shape key will ignore any existing data in that shape key. It will be as if you are sculpting a brand new shape key. When you leave sculpt mode, the "fake" shape key you've been sculpting will be added onto the actual shape key that you were meant to be sculpting.

Also note that the vertex group that is specified for the Mask modifier must have some verts assigned to it to produce the bug, even if the assigned weights are 0. The bug doesn't happen if no vertex group is selected.

Exact steps for others to reproduce the error

• Download attached blend
• Note how the currently active shape key turns the cube into a sphere.
• Enter sculpt mode
• The mesh turns back into a cube (as if the shape key didn't exist)

Workaround
Disabling the mask modifier while sculpting makes the problem go away.

I noticed this warning on the modifier when entering sculpt mode

But note that this warning existed in 2.7 as well, but in 2.7 Mask modifiers work perfectly fine when sculpting a shape key, so I find this warning odd in any case.