The bug is because AS' process depends on some action occurring to actually upgrade your seat. That action can be one of the following:
- Sweep at 72 hours for Gold
- Sweep at 48 hours for MVP
- Sweep at 36 hours for elites (supposedly new, not seen in action yet)
- User-initiated upgrade request during OLCI window
- User initiated request at purchase time (for eligible fares only)
- GA doing final sweep before boarding
All of the above will only be successful if there is U space actually available, which can happen at any time. The act of making U space available does NOT automatically clear those on the upgrade list. (If it's supposed to, it does so with shockingly low frequency.)
Once you know the rules, it becomes clear that it's critically important to either:
a) watch like a hawk for U space after 72 hours and call for a manual upgrade, or
b) use OLCI at exactly T-24 to secure your upgrade.
Tools like ExpertFlyer can give you invaluable insight into the available space in F and U classes and are highly recommended.