If you still want the reward ticket, and are willing to potentially pay for a OW from IAH-AUS, you can book the AUS-IAH-Mexico-IAH segments now. I think you should get a free stopover, so you can book the final IAH-AUS segment for any future date/time that has availability. You can then request to waitlist for the real IAH-AUS segment that you want. Of course, if nothing opens up, you'll have to separately purchase the final IAH-AUS segment. (Alternatively, if you're platinum, there isn't any fees for changing your award ticket after it is issued).

Anyway, I agree that the reward booking engine seems to have gotten worse recently.