System Information
Operating system: Windows-10-10.0.17763 64 Bits
Graphics card: GeForce RTX 2080 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 441.66

Blender Version
Broken: version: 2.81 (sub 16), branch: master, commit date: 2019-11-20 14:27, hash: rB26bd5ebd42e3
Worked: (optional)

Short description of error
Weight Paint and sculpt mode operates on the rest pose vertex positions rather than the deformed pose if a mirror modifier is above the armature modifier in 2.81

Exact steps for others to reproduce the error
Attach a mirror modifier to any model. Attach an armature modifier to that model with weights established. Deform or move the object in pose mode. Attempt to weight paint or sculpt the model while the pose is still in effect. In 2.8 this will work as expected and the vertices can be affected by their deformed positions. in 2.81 you will be unable to affect the weights by clicking the visual deformed location of the vertices, and instead, they must be affected by clicking the theoretical rest pose locations of the vertices. (options on the modifiers like OnCage and edit mode have no effect) (this problem only occurs if there is a mirror modifier in the modifier stack)

See in attached blend file. in 2.81 if you attempt to weight paint or sculpt on the cube in it's deformed position, you cannot affect it unless you click where the vertices would be in their rest pose. Open in 2.80 and there are no issues.