Might already be fixed, see MC-208617 and MC-208675

It seems like there is no transition between the axolotl's "swimming" (?) and "idle" (?) animation. This causes the axolotl to look very glitchy when it's starting to swim after idling for a bit, especially when swimming upwards or downwards.

This appears to happen especially often at the water surface.

Video

Minecraft 21w19a - Multiplayer (3rd-party Server) 2021-05-17 19-15-29_Trim.mp4

(In the video you see a water pool enclosed in barrier blocks)