Having Problem With Welding a Model

Hello! I’ve made a model in my game and I want to make an animation with it by using Moon Animator. Before, I didn’t even know how to rig the model properly so I made another Topic about it but ended up learning how to do it myself. I now know how rigging works though I came across this problem where when I try to weld a part to another, the rig reverses itself in a way and I can’t seem to move the welded part in any way. For more context, I’ve also attached a video that I’ve recorded and in the video I show how the rig is normally supposed to work and what happens when I weld the part to the unfinished rig.

(I uploaded the video to my Youtube Channel since DevForum can’t do it.)

Problem