Tang bones scaling.

[leucome]

The tang is a child of the head bones so it is not scaled with the face.
I think the tang bone could be a child of the face bone instead to prevent scaling issue.

The next part Is just a suggestion. I do not know if it is possible to inject new bones but if it is possible I would like more bones for the mouth.
I think 3 new bones per lips, one in the middle and on each side in between the corner and the middle. If it is possible it would help to create different unique mouth.

[ManlyMarco]

The bone structure is dictated by how the game was made and cannot be changed by ABMX. At best ABMX can lie about it and in the background do scale/position/rotation calculations to make the bone act as if it was parented to something else, but this will result in a glitchy result because of off-axis scaling.

[leucome]

It is a little bit unfortunate that it is not possible to edit the bones.
Yes, the off-axis scaling is noticeable. I tested just manually copying every transformation of cf_J_FaceBase manually it end up close but It need an offset to align with the mouth.