The {Team} NBT tag works for most entities including mobs, armor stands, but it does not work for the new Marker entity. The marker entity can still be added to a team with the /team command, however it takes more command blocks to do so and can interfere with other markers.

What I expected to happen was...:
The marker would be put into the team in the tag.

What actually happened was...:
It wasn't.

Steps to Reproduce:
1. Use the summon command with the {Team} NBT tag to spawn a marker in a team, for example, /summon marker ~ ~ ~ {Team:Ender}

2. The marker will not be put into the team, in this case Ender.

3. The only way to get the marker into a team is the /team command.