As I understand it, the issue here appears to be that, upon importing and then exporting an existing mouth model, the mouth in-game is distorted, generally with the upper area of the model being dragged down below the jaw line. The deformation carries over if the exported model is then reimported into Blender.
A few notes regarding the issue:
The broken part of the model always seems to be due to a bone being misplaced.
The bone in questions seems to be getting added to morphs that it should overwise not be used by.
Even if a mouth doesn't have a major bone issue, the model still becomes distorted in-game. This is most apparent with the Creepy and Cute mouths. May be related to the previously posted issue regarding Vertex importing / exporting.
This has been a recurring issue.
As I understand it, the issue here appears to be that, upon importing and then exporting an existing mouth model, the mouth in-game is distorted, generally with the upper area of the model being dragged down below the jaw line. The deformation carries over if the exported model is then reimported into Blender.
A few notes regarding the issue: