This kind of reminds me of a similar thing in other games, such as Ark Survival Evolved. In that game, creatures when no player is in render-distance of them would enter a mode of stasis and not move or interact or calculate anything. Then when a player enters that render distance and they exit stasis, it would update their stats like hunger and other stuff.
So I wonder if some of this may be by design with the Fortnite creature spawners. Can you try changing your render/viewing distance in game as a test to something like the lowest and highest settings and seeing if each of those settings changes the behavior of the spawners? I don’t have an idea about the solution but confirming that could help determine if it’s a resource-saving technique done by Fortnite’s system.