I found the cause of a quirk that had confused me for a little while: if you use the keyboard shortcut for Snooze on an email that’s already snoozed, the first press doesn’t re-snooze it — it un-snoozes it, resetting the timestamp back to the original received time. You then have to hit the shortcut a second time to actually apply the new snooze.
So on a previously-snoozed message the shortcut behaves like a two-step toggle (unsnooze, then snooze) instead of just updating the snooze time in one press. The first press looks like it did nothing, but it actually reset the message’s timestamp.
I’m not sure whether this is intentional, and now that I understand it I actually think the behavior makes sense, but figured I’d posting here in case it’s actually a bug, or in case anyone else runs into the question of having to press snooze twice.