I understand how layer and core priorities work, and my walls were correctly created to begin with, but it still doesn't explain why this crap keeps happening. Somebody can add a wall at the other end and that's enough to completely undo an hours worth of work forcing Revit to model a join properly. It can't even join two walls properly if there's an extension of the base or top.
I hear your frustration loud and clear. Let me take a stab at it. Post the file.
I cleaned up this corner (SE) but the other corner (NE - north is right in plan view) opens up in response. SE still looks funny and there's a small hole I can't get rid of near the low parapet. Thanks.