Weight painting in Blender 2.8 I am just starting to learn about rigging, animating and the like.

In Blender 2.8 I have a problem with a file that I cannot recreate but can not solve also.

When I am in weight paint mode while the armature is not in the rest pose it adds the weights as if it was in rest pose.
Meaning, if the fingers are bent I cannot paint weight on the bent fingers, but it adds the weight only if I paint at the (invisible) position where the resting armature would be. I hope that makes sense. Here is the file:



In the image you see me try tp paint weight on the pinky when it is bent, but it does not work. It only works when it is straight.



If I try it again with a blank file, it works fine. I suppose it must be one setting that I checked that is causing this.
Weight paint doesn't allow you to paint on vertices that were generated with the multiresolution modifier; it only interpolates the weights for each point based on its neighbors (same as the subsurface modifier). It also won't let you modify the mesh with any modifier besides the armature modifier while you're in the weight painting mode. 

A simple solution would be to simply apply the multiresolution modifier. If you'd like to retain the original (low poly) mesh, you could create a duplicate of that mesh, hide it, and apply the multiresolution modifier on the duplicate mesh.