This, although I swore at one point last year, when they loosened restrictions on ETC/FFC, I was given the option to use TB on a reservation that had an AC segment. I didn't end up booking, so have no idea what would have happened if I tried.

There's also been reports of it not showing up as an option when UAX flights are part of the itinerary. Seems not all the time, but sometimes. There may have been a similar issue a couple of years or so ago. I have not done a lot of bookings recently, but have, when looking, seen the option, even on all UAX itins. If using regular website, can try using the legacy payment page (there's a link by payment options) to see if it shows there. I have also seen it as an option for sure on the app when a UAX segment was there as well.