No, my “sense” is that the user wants to use RootsMagic to land the contents of their Family Historian database and its associated record identifiers (for each individual) up onto Ancestry.com as a tree (ie. use RM to do what FH can’t do on its own).
But, when RM11 imports the GEDCOM (v 5.5 from Family Historian) with its _UID tags, they (the FH record identifiers) are not making their way to Ancestry via the TreeShare API (despite being intact in the PersonTable UniqueID column). To attempt to prove that RootsMagic introduced this new problem, they go one step further and export a GEDCOM down from Ancestry (after the TreeShare UP) and there is no longer record identifiers represented by either _UID or just UID GEDCOM tag(s).
I suggested perhaps because the API for TreeShare UP might have transmitted up the record identifiers in the form of the more common 1 _UID tags, instead of the way Ancestry supposedly deals with them …which is 1 UID (no underscore), thusly making them “not accepted”.