Reimporting Mannequin Skeletal Mesh warps animations

I am following this tutorial( which is five years old at this point so maybe that’s the issue) but I’m trying to make an armor piece from the mannequin rig for prototyping a modular pawn mesh that I’ve read about. First thing I did was simply to export the Mannequin mesh then load it in Maya 2018( of which I know nothing about) and then tried immediately reimporting it back into maya. Here’s the result:

I’ve troubleshooted this for hours and still no fix. The A pose looks correct, but when I apply an animation the left shoulder becomes detached. I tried the steps in this [link:][2] but to no avail. I can’t get the shoulder to be normal. Any help would be appreciated!