Axolotl's render incorrectly based on your head rotation. Check the images.

The issue:
Axolotl's when alone in the water, won't render any axolotl's that are under the water.

How to replicate:

• Turn entity shadows off (Possible with them on, but happens much more frequently when they are off)
• Spawn Axolotl's in an enclosed area of water.
• Try changing your position and head rotation