There have been other reports of downgrades which seem out-of-line with any logic, including pax on paid upfares to F being downgraded in favor of GMs who bought upgrades (at TOD rates) at check-in.

I think there's something seriously wrong with UA's downgrade logic in SHARES. Some GAs have blamed it on time of check-in--I saw that in person on an equipment swap a couple of weeks ago--yet we have had several reports like yours of pax who checked in at T-24 getting booted. There's no way everyone in F checked in at T-24.

I suspect what's happening is that SHARES isn't prioritizing downgrades and DBs due to oversales in any sort of outwardly-deterministic manner. We at PMUA were used to it being done by status, which seems to be an anathema to SHARES.