And there's no guarantee this won't happen again. Partner awards are notorious for this, especially TG.
My advice..check your reservation like a hawk daily or at least weekly. And make sure you save the email ticket receipt which shows a valid ticket number and flights with cabin classes that you're booked into. As someone else pointed out, TG/UA only have to get you to your destination but are not required to do so in the cabin you booked without refunding you the difference.
Glad to hear TG opened the space for you. Closer to flight time this is highly unlikely to happen. I hope the flights don't fall off your record again.
-RM