Is the Sent or Sent Items folder, whatever it's called, a subscribed folder when you right-click the (IMAP) account in the folder pane, Subscribe, click Refresh? With Hotmail/outlook.com accounts, the 'Place a copy in' box should be unchecked in the Copies & Folders section of Account Settings since sent messages are automatically copied to the Sent folder, so I presume the same for Office365 accounts.

That did not work.

How do I recover the sent email history is the real question.

Did you back up the mail folders before switching the account? If you can't subscribe to a folder for the new account, open the profile folder and see if there's a subfolder of ImapMail that contains the old account's mail. Help/Troubleshooting Information, click Show in Finder to open the profile in Finder. Look for a file named Sent, with no extension, in a subfolder of ImapMail that is named after the account. Copy the Sent file to Mail/Local Folders (with TB closed), and it should be visible under Local Folders when you restart TB.

There was a delayed reaction to the Subscribe process before it finally took, maybe even after rebooting. I can see all the old sent emails again - thanks for the suggestions.