To reproduce, do these commands:
/scoreboard teams add testTeam1
/scoreboard teams add testTeam2
/scoreboard teams join testTeam1
/scoreboard teams join testTeam2
/scoreboard teams leave testTeam1 @p
/scoreboard teams list/scoreboard teams add testTeam1
/scoreboard teams add testTeam2
/scoreboard teams join testTeam1
/scoreboard teams join testTeam2
/scoreboard teams leave testTeam1 @p
/scoreboard teams listExpected: No players in testTeam1 and 1 player in testTeam2
But instead there are no people in any of these teams
You can only be in 1 team at a time.