For anyone else coming across this thread, I wish I could tell you what happened and why it’s working now. In the past, because I’ve been burned, I’ve created duplicates of important assets like skeletons. In this case, the duplicate was only messed up with the issue I mentioned earlier about retargeting to “Skeleton” instead of “Animation”.
So, fortunately for me, I was able to use that skeleton instead (and create a duplicate of IT!)… The only thing I found by myself on this topic was the “ik_hand_gun” bone name seems to be what is causing this issue. However, when I deleted these bones it’s still happening so I’m not sure what’s going on there.
Best of luck, sorry I couldn’t help more.