The bug

Despite invisible players or invisible squid not being attacked by guardians, guardians will still attack invisible axolotls.

How to reproduce

1. Construct a pond of water (or go to an ocean)
2. 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}]}

3. Spawn a guardian within the water as well:

   /summon guardian

   The guardian still attacks the axolotl, despite the axolotl being invisible

4. Repeat the steps above, but replace axolotl in the first command with squid
   The guardian does not attack the squid (the axolotl may attack the squid, however, so pay attention to what damages what!)