Mod Version: 3.1.2
Game Version: 1.16.5
Forge Version: 36.2.39

Console is getting spammed with attempts to add a thrasher but it's already been marked removed.

[05:52:21] [Server thread/WARN] [minecraft/ServerWorld]: Tried to add entity upgrade_aquatic:thrasher but it was marked as removed already [05:52:21] [Server thread/WARN] [minecraft/ServerWorld]: Tried to add entity upgrade_aquatic:thrasher but it was marked as removed already [05:52:22] [Server thread/WARN] [minecraft/ServerWorld]: Tried to add entity upgrade_aquatic:thrasher but it was marked as removed already [05:52:22] [Server thread/WARN] [minecraft/ServerWorld]: Tried to add entity upgrade_aquatic:thrasher but it was marked as removed already [05:52:22] [Server thread/WARN] [minecraft/ServerWorld]: Tried to add entity upgrade_aquatic:thrasher but it was marked as removed already [05:52:22] [Server thread/WARN] [minecraft/ServerWorld]: Tried to add entity upgrade_aquatic:thrasher but it was marked as removed already [05:52:22] [Server thread/WARN] [minecraft/ServerWorld]: Tried to add entity upgrade_aquatic:thrasher but it was marked as removed already [05:52:22] [Server thread/WARN] [minecraft/ServerWorld]: Tried to add entity upgrade_aquatic:thrasher but it was marked as removed already [05:52:24] [Server thread/WARN] [minecraft/ServerWorld]: Tried to add entity upgrade_aquatic:thrasher but it was marked as removed already [05:52:27] [Server thread/WARN] [minecraft/ServerWorld]: Tried to add entity upgrade_aquatic:thrasher but it was marked as removed already [05:52:35] [Server thread/WARN] [minecraft/ServerWorld]: Tried to add entity upgrade_aquatic:thrasher but it was marked as removed already [05:52:36] [Server thread/WARN] [minecraft/ServerWorld]: Tried to add entity upgrade_aquatic:thrasher but it was marked as removed already

This is a generic vanilla issue that can happen for several reasons. Thrashers do nothing that causes them to get added back to the world after they get removed.