This issue turns out to be very closely related to one as I reported earlier (button in portal not firing); but this version/description of the issue has a broader scope.
OK, did finally find it in the new system: Using 'hide object' clauses in a portal row causes button in the 'Create new record' row to not...
This is the same issue as the other link. In essence, a button in a portal row does not function if an active portal field has a "Hide object when" condition.
Yes, it is the same root issue, but I wanted to highlight the fact that the problem wasn't restricted to just portals, it also happens in list views. Probably anywhere that the FM engine wants to switch the 'active' record.