It could be that they are subject to the challenge rating of areas. Initially implemented and after updates its likely that their spawns are pretty flashy when their numbers have been put up to max. Last time I travelled I found two mist "spawns", one didn't appear to do anything beyond spawn in mist placeables, the other was a singular "Mistling" that came out.
I think there's a factor in play that's reducing the threat, and I think people prefer them when they are at their "maximum threat level" but conversely, there are threats in there that will just outright bully low levels (Greater Will-o-wisps?).
If there is a bug with what pops out along with placeables etc, then I hope that is fixed. If it's not a bug but a feature, then I would hope it's updated more to fit the level of the player who triggered or is closest to the event so it's always a heavy handed but polite warning that the Mists are not to be trifled with at night.