This sounds like it could be a problem with the poiyomi toon shader. That shader has a tendency to break when different versions are installed on top of each other. You may want to re install the poiyomi shader.
but it is not JUST the shader its also some clever modeling/viseme configuration on my part. The shortest way I can explain what it is doing is something of a non-ZTested portal effect. For the mouth at least, there is a material set to a mask that simply hides the head material and reveals the mouth material sitting just behind it, and the visemes mainly control the shape and size of that mask to give the illusion of a mouth opening.
No, really it IS Poiyomi's toon shader. But to be more specific its using the stencil function that the shader provides. Though it is totally possible to modify other shaders to use Unity's stencil buffer. This is the chunk of relevant documentation -> https://docs.unity3d.com/Manual/SL-Stencil.html. While Poiyomi has this functionality by default I can hack existing shaders to use the buffer.
He was talking about this sites general purpose tutorial for getting models to be avatars. you may wana look into having to manually set avatars up, it will help you a lot on this site.