The bug
Despite invisible players or invisible squid not being attacked by guardians, guardians will still attack invisible axolotls.
How to reproduce
Construct a pond of water (or go to an ocean)
Run the command below while in the water (the Glowing tag is only to ensure that you know where the axolotl is):
/summon axolotl ~ ~ ~ {Glowing:1,ActiveEffects:[{Id:14,Amplifier:0,Duration:999999}]}Spawn a guardian within the water as well:
/summon guardian❌ The guardian still attacks the axolotl, despite the axolotl being invisible
Repeat the steps above, but replace
axolotlin the first command withsquid
✔ The guardian does not attack the squid (the axolotl may attack the squid, however, so pay attention to what damages what!)
Can confirm.