With 2 players and 4 teams we are unable to move a player from team 3 to team 2 and team 4 to team 1. We are however able to move players between team 1 and 2 as well as 3 and 4. In all of these cases the target team already has a member.
Check your Island Settings device for the Teams, Team Size properties. Are you set to “Even Split” and potentially the operation fails due to unbalanced teams?