System Information
Operating system: Win 10
Graphics card: Geforce GTX760
Blender Version
Broken: 2.81
Short description of error
In Weight Painting, the brush seems to paint my character as if it were in rest mode, while I was in pose mode.
The solution seems to deactivate the display of any modifier that is on top of the armature modifier.
Exact steps for others to reproduce the error
Add a modifier (here it was the EdgeSplit modifier) on top of the armature modifier, then try to paint the weights in Pose mode.
Note that some people reported the issue with other modifiers:
https://blenderartists.org/t/weight-painting-not-working-properly-while-model-is-posed/1181329/4
Hope that helps!