I think this just affected Guardians but this affect all mobs.

If you spawn a Zombie with specific team (for example: BLUE) and you spawn other Zombie with other team (for example: RED) They'll not start to fight or attack each other.

I see the mobs can fight or attack each other but tney need to be provoked.

The only mob start a fight with other teams without provocation are the Shulkers.
The Wither also can start the fight but this mob also attack the mobs in the same team.

Steps to Reproduce
1.- Create two teams at least (/scoreboard teams add <TEAM1 | TEAM2 >)
2.- Summon a Shulker with Team Tag ( _/summon shulker ~ ~1 ~

_ )
3.- Summon a Zombie with Team Tag ( _/summon zombie ~ ~1 ~

_ )
4.- See the Shulker will start attack the other mob. And only when the other mobs gets hurt it tries to attack the Shulker

I think this is a issue because Shulkers can start a figh with others teams, so why the others mobs can't?

The list of the mobs affected are this: (I tested)

Blaze
Cave Spider
Creeper
Elder Guardian*
Endermite
Evoker
Ghast
Guardian*
Husk
Magma Cube
Shulker*
Silverfish
Skeleton
Slime
Spider
Stray
Vex
Vindicator
Witch
Wither Skeleton
Zombie
Zombie Pigman*
Zombie Villager

Also if you spawn a passive mob in a team and you spawn a hostile mob in other team, the hostile mob don't try to attack the passive mob.

*Guardians and Elder Guardians even if they're provoked do not attack. I test this with a Villager golem and a Shulker, The guardians do no attack they. The just attack Squids and Players.

*Shulkers for example can attack hostile mobs. But they don't attack passive mobs in different team

*Zombie Pigman also needs to be provoked. But I think if you have a Yellow team Skeletons army, and someone It a Red Zombie Pigman Army, The Red can attack all yellow army, no only try to fight just one mob (this is a suggestion)

Villager Golems attack hostile mobs but no passive mobs
This also happen with the Snowman

Sorry for my bad english
try to make this as complete as possible

Sorry if this is a replica, but I search and I don't found other report.