I can explain why your observation is believable, with one caveat: no watchOS 27 release note I have seen calls this fix out by name, so it rests on the behavior itself.
The background: the watch has no time zone setting of its own. It mirrors the iPhone, including when you turn Set Automatically off on the phone and pick a zone manually. The old misbehavior was in the disconnected state: once the watch lost its phone connection it fell back to deriving the zone itself from its own location and network information, which snapped it back to local time exactly as you describe.
If 27 keeps the manual zone while separated, the disconnected fallback is now respecting the synced setting instead of re-deriving, which is what a fix would look like. Two ways to prove it conclusively on your Ultra 3:
1. Restart test. With the manual zone set on the iPhone, turn the phone completely off, then restart the watch while it has no phone to talk to. A reboot forces the watch to rebuild its time state from scratch, so if it still wakes up on the manual zone, that is stored behavior, not leftover state.
2. Cellular off test. On a cellular model, leave the phone behind with cellular also turned off on the watch, so it cannot pull time from the carrier either. If it holds the manual zone through that, the synced setting is genuinely in charge now.
Worth posting your result back here either way, since yours is the first report of it I have seen.
User level: Level 2 (202 points)
