They indeed have several apps called Teams, 2 of them are incompatible by design: the home and the business version. Both of which probably have different active versions by now, all called Teams. And the user doesn't know which one they have open.

What's broken with Microsoft changes over time and I haven't used Teams in a good while, but at one point it was possible to send someone a teams meeting invite, they would download teams from the link in the mail and get the home version unbeknownst to them, leaving them unable to join. Microsoft would decide which version the user got based on their Windows account, regardless of the invite.

One solution for this was to find a direct download link of the right version of Teams, which was posted on some trouble shooting page. But even with the right version, the user could still be automatically logged into that teams with the wrong account. For meetings outside the company, it was just easier to not use teams.