In Nuka-World junkyard, there is one protectron enemy that spawns below the map (in the highly-radiated mound in the water south of the barn), which makes it impossible to destroy, unless you fast travel away then fast travel back.
In normal circumstances, I would have put the severity of this to very low instead of low. However there seems to be an additional bug regarding the enemies in the Nuka-World junkyard. It seems that the enemies aren't supposed to respawn after you clear the junkyard, and that happens most of the time. However I noticed that if you leave one enemy there alive for a while, then come back all the enemies there will respawn repeatedly and indefinitely instead.
I do not believe this issue is tied to the quest "Trip to the Stars" where you need to clear the junkyard for the Hubologists to move in there, but am not100% sure.