@Mach Admin - @Kiron Bondale is correct - once a member leaves a team, the boards/projects are either transferred to another user or deleted. Even if the same member is added back with the email address/account they used to create the boards, the ownership is still lost.
There are a number of options for these users to get the boards back under their ownership:
- If there is a manageable number of boards, before their email address/account is removed from the team, they could download a backup of the board and then restore them from their other account.
- They could first transfer ownership of the board from the account that is about to be removed to the account that is staying.