I have solved the problem by doin' something quite radical. I have completely removed all default clothing and the default FX layer, added all the dressing sets I needed and used VRCFury + MMD fix from VRCFury to create an FX "on the fly". This solved the back to default clothes (or worse the naked Chalo dance).
It's an FX layer problem, and Mofuaki never fixed it.
@crispycrem i sent the smutbase link in the comments, you'll be able to grab the full thing from there
it has an official blender file rather than the one i slapped together
The archive you linked has PDFs with instructions, if you need translation - you can use this website: https://pdf.translatewebpages.org/pdf.js/web/viewer