Yeah I’d be annoyed too.
You go through the whole booking flow, pick your seats, everything looks confirmed… then suddenly one leg turns into “assigned at check-in” with no way to change it unless you pay more. That feels sneaky.
I don’t think it’s because you used points. Points bookings are usually treated the same as cash. My guess is either the seat got reallocated due to an equipment change or they’re tightening up inventory on certain rows and pushing people toward paid upgrades.
Still, if you’re A-List, the expectation is you’re not getting bumped into a random middle seat at the back.
If it were me, I’d call and ask them to explain exactly why it changed. Sometimes a rep can manually fix what the system messed up.