Debug profiling of the issue. All datapacks were removed to make sure this was a purely vanilla issue
Still persistent in 1.20. This also applies to axolotls who are already in water, they will try and path find to bodies of water that are out of sight. Can cause axolotls to group into a corner of a fish tank under the right circumstances if a pond is nearby.
Update: was an issue related to a deleted datapack. Was fixed by killing armor stands and particle effect areas.