Operating system: Windows-7-6.1.7601-SP1 64 Bits
Graphics card: GeForce GTX 1070/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 430.86
Broken: version: 2.82 (sub 1), branch: master, commit date: 2019-10-28 21:19, hash: rB2dab4393b08e
Broken: hash: 2.81 95f020c853d6, build date: 2019-10-14, 00:11:29
Worked: f88022b96f3a build date: 28/08/2019, 01:48
Short description of error
If an object has an armature its normal to move bones to see the effect and paint on the deform mesh, this always worked.
In the lastest builds it seems that adding a mirror modifier after the armature modifier mess up how the object its read and the stroke doesn't register that the mesh its deformed.
Exact steps for others to reproduce the error
1-Open the blend file
(it was tested using default factory settings too but it was created with my normal set up, you may see some addons active, as i said i also tested in a default settings)
2-try to paint in the blue circle(1), nothing should happen.
3-try painting in the red square(2) it should paint, as if the cylinder wasn't deformed.
4-Reload the .blend file
5-delete the mirror modifier and repeat step 2 and 3