When a zombie is underwater and a player is in water or above land the zombie will try and jump up blocks to get to the player but it will only jump up halfway preventing it from getting over the block.
MC-129601
Zombies can only jump up blocks halfway underwater
Resolved
Fixed
2
Roberto Smith
(Unassigned)
Community Consensus
Minecraft 18w19b, Minecraft 18w20a, Minecraft 18w20b, Minecraft 18w20c, Minecraft 18w21b, Minecraft 1.13-pre1, Minecraft 1.13-pre6
Minecraft 1.13.1
Retrieved
Still an issue in 18w20a