So I'm following the tutorial on exporting my CC3 character to Substance. Everything I've imported has worked correctly except for the helmet, which looks super shiny. This is just the normal map. Nothing else. The normal detail is in the right place, but maybe it's reversed somehow? I can't figure out how to fix it. 76% of original size (was 661x19) - Click to enlarge

So it looks like what had happened was (at last in part) my error after all. I had imported the helmet into CC3 in pieces, rather than letting it merge into one asset. I went back to CC3, deleted and reimported, and put that new OBJ back in CC3 and it was fine.