This bug report is a parity issue with Java.

Summary:
Water splash potions don't give the axolotl 1.5 minutes of moisture.
This bug has been fixed in the Java version MC-221501.

Steps to reproduce:

1. Obtain Axolotl Spawn Egg and Splash Water Bottle.
2. Spawn Axolotl by Spawn Egg and wait for 5 minutes.
3. As soon as the axolotl starts taking damage, immediately throw the Splash Water Bottle at the Axolotl.

Observed results:
Water splash potions don't give the axolotl 1.5 minutes of moisture. Other sources like Water, Rain and Bubble Colums fully moisturise Axolotl.

Expected results:
Water splash potions also give the axolotl 1.5 minutes of moisture.

Video:
Minecraft 2021-04-12 17-36-23.mp4