There are definitely bugs in the system, but I understand AS is working hard to swat them.
I purchased an H class fare when U was not available, and later was notified by ExperFlyer that U became available long before my flight date. I was supposed to be immediately upgraded (AS policy for H or higher class purchases for MVPG), but it didn't happen. So I called and AS manually pushed it through.
I reported this to AS, with screen shots, and they told me they're trying to figure out what went wrong in my case. It's possible I would have been automatically upgraded at some point after when I called, perhaps within hours.
But I think keeping an eye on U space, using ExpertFlyer or another service, is never a bad idea. Just in case.