One of the worst offenders might be the Sports shop in building world return, where if you wait too long
Now, when re-playing (gathering those elusive field texts) I've had hummers spawn out of nowhere in a couple of forest rooms. Hummers I don't think were there before... Again, it took maybe a minute or two before they appeared, long after I had killed other enemies in the room.
Or, since this is an easy-mode re-romp, some enemies just naturally take a longer time to spawn? Have you had experience with something like this?