Now, why could the forum search engine not find my comment above ?? D'uh...

Belatedly thinking laterally, there's another possible explanation for missing textures: Some MMD PMX artists habitually set their models' texture folders to 'hidden' in Windows' File Explorer. ( They'll also 'mung' the PMX geometry which, IMHO, is a different bucket of stinky 'chum'...)

If an OBJ+MTL artist did this, and original path was 'baked' into OBJ+MTL, you would have to edit those paths and un-hide such covert folder(s)...