Is this content for which there is only a bump map for the head? CC3+ characters include part of the neck in the head texture, so if there is no body bump map, issues may crop up.
Just speculating, because thus far I have had no conversion problems, but then again the concerned characters all had normal maps.
EDIT: Never mind, there is a much better explanation here:
https://forum.reallusion.com/FindPost451489.aspx.