Body moves relative to arm when animating custom rig that has been welded

I’m trying to make a monster model that is animatable.

I made a basic custom rig model and nothing is wrong with it (I tested it in the animator) but after I welded parts to it and I try animating it, I can’t rotate the right arm because instead when I do, it rotates everything else, relative to the right arm. I tried welding the parts to the rig and the rig to the parts, it still has an issue with the right arm and occasionally moves the legs downwards. It’s as if the right arm is the main part and if it is, I don’t know how to change it. Without the welds, everything works fine.

Moving right arm with rig welded (not working):
Screen Shot 2021-08-08 at 6.21.05 PM

Moving right arm without any welds (working):
Screen Shot 2021-08-08 at 6.21.37 PM