(Warning, techno-babble ahead. If you don't understand what it means or why it's relevant, you most likely don't need to.)

So, replacing the Antonia 1.3 source OBJ with a welded version exported from Poser seems to have worked nicely. Will do the UDIM conversion next. Slightly inconvenient that Blender still doesn't seem to have face group support, but I can work around that.

-- I'm not mad at you, just Westphalian.