It's also quite possible the bug is Apple's issue, not United's. The (non-travel related) app my team wrote is broken on iOS12 now. Worked great in betas but Apple broke something in the full release of the code. Apple has admitted it's their bug and are working on fixing it. That means 12.0.1 will likely have the fix.
If United is in the same boat they are either scrambling to find a work-around or they're just waiting for 12.0.1 so they don't have to waste so many hours trying to code a change that will be fixed by Apple soon.
Just a guess. I don't know if this is truly an Apple bug causing United's issues but it's highly possible.
-RM