I've noticed the same thing on SEA-BOS - my upgrade consistently clears at T-24 and when I check inventory at that point, it suddenly has gone to F0.
It's really nice for me because AS pre-order closes 20 hours before departure, so it gives me a couple hours to select my meal. However, I can see how it would be really annoying for SDCs or people in paid F who run into IRROPS.