The following two commands work:
/scoreboard teams join TEST @e[type=Item]
/scoreboard teams list TEST
(will successfully display item entity's UUID in chat)

However, commands that use the "team=" selector fail:
/say @e[type=Item,team=TEST]
This also happens for items not on teams.
/say @e[type=Item,team=!TEST]

Entities I have found to be affected by this:
Items, Armor Stands