It's a problem because X does not get you on the CPU waitlist whereas XN does. And by your ambiguous question I can't tell if you realize or not so just to inform, award tickets booked with United credit cards are eligible for CPU.

Of note, my previously mentioned X class ticket did not get me on the CPU waitlist but did not bother to argue it at the airport because F was already full.

So to summarize: booked XN, inherited 'up arrow' CPU-eligibility (correctly so), lost up arrow, XN somehow converted to X, no CPU and not on upgrade waitlist at the airport.