When in a biome that is eligible for Thrasher spawning, the titular warning pops up in the log rather frequently. I do not know exactly why it's showing up or what it means, but it is probably unintended for it to be happening so often - thus I'm making this issue.

Minecraft 1.16.5
Forge 36.1.4
abnormals_core-1.16.5-3.1.1
upgrade_aquatic-1.16.5-3.1.0

There isn't really much to work with here, as this a normal error that can happen with any entity for a variety of reasons.
It may be caused by an excessive amount of thrashers being in an area. Try lowering the spawn weight in the config.