We had a fake domain and a productive domain and we made a hard link connection between the AD users and o365 users! It is very cool!
But after linking, we faced with following problem.
We can't open files in Teams Chat that started earlier!
Is there a solution for this?
I'm not sure about the setup of your domains. But the files you share in a Teams chat will be uploaded to OneDrive, so if you change domain or user accounts for someone in the chat the permissions might changed if there are new accounts created.