I believe resource packs from 1.14 are not compatible with 1.15, they have changed format.
I believe this issue represents the fact that their pathfinding doesn't avoid water, causing them to unintentionally drown themselves.
Isn't this irrelevant because it is a modded client?
I'm am experiencing the same issue on snapshot 19w45b.
Similar error for me on 14.4
Can confirm, occurs on multiplayer, too.