Why is a new employee seeing old group chats from a previous job?

First off, make sure the new employee is only logged in with the new company account. It's easy to accidentally sign in with a personal or old work account without realizing it. You might want to double-check that nothing else is logged in on their device.

I faced a similar situation once. A new hire had joined us, but they used to work as a vendor and had some previous chat history still in the system because they had contacts in the tenant. It could be worth looking into if this employee might have had previous interactions that got synced back when they logged in.

It sounds like this person may have somehow used their old credentials while signing in, or they could have been inadvertently invited to that chat. I know it seems like that shouldn't happen, but sometimes when people transition jobs, old contacts remain in the system.

What if an old colleague added them to the group chat before they even logged in? It's a long shot, but if someone from the old company kept in touch, they might have linked them up without the new hire's knowledge.

Have you checked if the device is somehow associated with the old company's management software? If their old accounts were on that device, it might be pulling in some of that old data. Also, verify that their personal accounts aren’t logged into travel apps or any browsing accounts.