The bug

The Team tag makes an entity directly join a team once its NBT data is read. As all entities can be in a team this tag should exist for all entities and not only for the ones extending EntityLivingBase.

How to reproduce

1. Create a team

   /team add testTeam
   

2. Summon the following entities

   /summon armor_stand ~ ~ ~ {Team:"testTeam"}
   /summon minecart ~ ~ ~ {Team:"testTeam"}
   

3. Run the following command

   /team list testTeam
   

   → It only shows the armor stand

4. Run the following command and repeat the previous step

   /team join testTeam @e[type=minecart,limit=1]
   

   → Now the minecart is listed as well