It has happened again. Ignoring how inconsistent the weighting is, MMD is treating any vertices I painted for butt physics as if they were weighted to a phantom fifth bone that A) doesn't exist and B) is higher than every other bone (note how the vertices still go up despite every bone lowered).

Annoyingly, it looks perfect in pmxeditor, even in transform view with physics/play/motion enabled. It strictly starts going haywire when loaded into MMD. I can't see anything wrong when I'm inspect the data of the vertices individually...