Replicate:
/scoreboard teams add Test
/scoreboard teams option Test friendlyfire false
/scoreboard teams join Test
/summon Skeleton ~ ~ ~

/summon Skeleton ~ ~ ~

Getting the Skeleton to shoot the skeleton on my team doesn't attack the skeleton nor I. Also attacking the other skeleton the skeleton on my team doesn't attack.

Same result with other mobs as well. Basically they do nothing.

Testing if 2 skeletons without the team tag attack each other if they shot one another with a arrow and they do

Testing if zombie targets a villager and it does even with a team tag.