You most likely have the armature displayed in the original rig file so when the bones are selected, they are z-fighting to show on screen.
This is another one of the 2.8 issues that they have not yet fixed (and probably won’t for proxies)
But as a work around, go to the rig file and turn off the visibilty in all view layers (the computer screen icon not the eyeball), then save the file. When you go back to the animation file, you will only see the proxy version of the rig so pose mode will look normal.
I think that is what you mean – let me know if I am mistaken.
