The Bug

Like wither（it has been fixed in 1.15 snapshot）, puffer fish will attack any mob except the same kind mob(undead or aquatic life), even if their target is their teammate. Other mobs won't do that.

Steps to Reproduce:

1. Create a team called "a"

   /team add a

2. Close the friendly fire of this team

   /team modify a friendlyFire false

3. Join this team

   /team join a @s

4. Summon a puffer fish of "a" in the water

   /summon minecraft:pufferfish ~ ~ ~ {Team:"a"}

5. Change your gamemode to adventure or survival
6. Approach the pufferfish
   -> The puffer fish puffs up and poisons you, unlike most other hostile mobs in your team.