I got the following reply from trello support.
Thanks for writing to us! Sorry for the trouble.
The Microsoft Teams team that develops and maintains the integration recently made a change that removed the top bar from Trello boards in Teams. I'm afraid that it's no longer possible to search for other boards within teams. The devs have recommended that instead of searching, users add new boards inside of Teams as a new tab.
I'm so sorry that I can't do more to directly assist here. Please let me know if you have any additional questions or concerns, and I'm happy to help.
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