Text-based conversations between users with searchable history in Microsoft Teams Free
The search function doesn't work in conversations migrated from Skype, but should work in native conversations created in Teams.
Microsoft has confirmed that this is a known issue, but there is no ETA on when this issue will be fixed.
"You are correct, the in-chat search does not work correctly in some migrated Skype chats. This is a known issue and we are working on fixing these chats. It's just there's a lot of Skype chats and the process continues to take some time. And unfortunately there is no immediate workaround available for the affected chats except to wait. Creating a new chat could be an option too, but I do understand this is not the best way if there's extensive chat history in the old chat."