An assignee is not automatically removed, once he/she is out of team (BB-10959)
When you remove a person from a team, he/she is not automatically removed from all tasks, that were previously assigned to him/her. You can still see him/her in task details:
You can seem him/her removed only, when editing particular task:
Private comment: Problem is fixed, when task is saved (assignee automatically removed). But... God! How many of bugs like that you have there? Either Bitbucket is still in very early alpha stage or you don't pay your test team too much. Because bugs such basic, like this one, should be captured and removed before website or service is going to public. Yet, it seems, that you have quite a lot of this-class bugs, though Bitbucket is for years on market.