When running the following:
/scoreboard teams join TEAM1 @e[type=Item]
The item entity is entered into the team as expected. However, inserting a "team" parameter like so:
/scoreboard teams join TEAM2 @e[type=Item,team=TEAM1]
Will not work, and the entity remains in TEAM1. The same occurs when setting the parameter to "blank", "not blank", and "not on team" (given that they could match to begin with, of course):
/scoreboard teams join TEAM2 @e[type=Item,team=]
/scoreboard teams join TEAM2 @e[type=Item,team=!]
/scoreboard teams join TEAM2 @e[type=Item,team=!TEAM1]
It will work correctly on player targets and regular mobs. Note that this is not restricted to joining teams (in which the following will not work):
/say @e[type=Item,team=TEAM1]
Linked issues
is cloned by 1
is duplicated by 9
Comments 1

Only living entities can be on a team, items are not living entities, only mobs, animals, villagers and players are.