Operating system: Windows 10 64b
Graphics card: Nvidia 1060
Short description of error
If a mesh has a Basis shapekey and second or more Shapekeys, as well as a Generate Modifier like Subdivision enabled, any time you try to sculpt on the second Shapekey you can only move vertices by clicking on the original Basis position, even if on screen or in Object mode the vertices appear in the new sculpted position.
This happens in all shading modes, only being able to click on the original Basis position to sculpt your shapekey, but is visually different in Workbench vs Lookdev/Eevee:
In Workbench, the newly sculpted shape visually resets to the Basis position as soon as you stop the stroke of the brush, You can only see the shape outside of Sculpt mode. (the modifier is also disabled in Sculptmode with Workbench but that's another bug that has been reported already, it might be related though). In Lookdev however, you can see the actual sculpted shape of the second Shapekey, but still, you can only click on the original position to move the vertices you want, which doesn't match visually.
Exact steps for others to reproduce the error
Based on the attached file:
Go into Sculptmode and try grabbing Suzanne's ear by the red part. If you do it from Lookdev you can see how you can only grab the ear by clicking inside the Annotation I drew. If you do it from Workbench you can see how it resets after each brush stroke on top of that.
Based on Starttup file:
Add a Monkey mesh object. Add two shapekeys to it. Pin and highlight the Key 1 shapekey. Add a Subdivision or a Mirror modifier. Go into sculpt mode and drag the ear. At first it seems fine since the position of the vertices is the same in both shapekeys when you create them, but the further apart you move the vertices from their original position, the more jarring it is to sculpt them.