The core issue pertains to axolotl behavior when standing on waterlogged stairs or slabs arranged such that they can walk/swim in the water. When standing above the block level of waterlogged stairs, axolotl will often stop swimming and start looking towards the player. When the player moves to the same or lower z axis position relative to the stairs, if they are close to the axolotl (within 5 blocks or so) they begin to swim as normal again. My hypothesis is the issue has to do with the "angle up towards the player" animation embedding their tails in the bricks.

The attached image shows a setup in which the bug occurs. If you spend enough time with the chunk loaded eventually all of the axolotl in the room end up stuck on the slabs. Jumping into the pond (not the waterlogged slabs/stairs) they immediately begin swimming again