Operating system: Win10 64-bit
Graphics card: GeForce GTX 1070
When changing pose bone selection of linked and proxied armature original armature is displayed simultaneously until bone is manipulated.
Might be intentional but feels distracting. Or just overlooked because of proxy to be replaced by static overrides.
Original armature IS displayed all the time (greyed out), also in earlier blender versions. It was only visible if there was a difference in Custom Shape (Object) property between original and proxied armature. Otherwise was original armature aligned with proxied one all the time and covered with that thus making it invisible.
So the problem is that original armature pops in to initial pose when bone selection of proxied armature is changed and updated correctly again when transformed.