Avatar Skinned Mesh Renderer Warped on WebGL

I have an avatar that I have imported into Unity as an .fbx file. The avatar has a generic rig. For this application I am working on the main use case with this avatar is to rotate the upper and lower arm bones to track motion.

Inside the Unity Editor and on iOS platforms the avatar looks great (see first screenshot). However for some reason on WebGL the mesh is a bit distorted around the shoulders (see second screenshot). It looks like on WebGL some of the vertices of the mesh are getting stuck in place. From my debugging I am almost certain it has nothing to do with the bone positions/rotations (in other words the position/rotation of each bone is identical between all platforms).

Any ideas what could be going on? Any tips on how I could debug this? Thanks!