Note: 1.12.2 is the one affected but the list does not let me put any other version other than 1.13

Utilizing the team feature, I summoned multiple mobs on the same team named "Castle", just to test this feature,

working as intended,the mobs did not attack me, who was in their team, nor other mobs on other teams, but they did attack players that Werent on my team, As said before, Working as intended,

Ive even summoned Zombies and villagers together on the same team, they did not react to eachother, they just hanged out together, working as intended,

until i saw this, I created a iron golem in the team, "Castle", summoned him, then summoned a skeleton on the same team next to him, he didnt attack it,

then i summoned another skeleton which the golem proceeded to attack, despite him beeing in the same team, them started spinning in circles,

Summoning the golem BEFORE the mob: makes the golem not attack the mob, atleast not instantly, they end up attacking them after a while anyways

summoning the golem AFTER the mob: makes the golem instantly attack the mob, the mob does not defend itself, working as intended,

I also tested this behavior with snow golems, and they do not repeat this behavior , snow golems, as intended, Will not attack friendly team members regardless of summoning time 

I made sure to check that they were on the same team, and it proved positive,