This just happened to one of my clients, they hate the new version and the forced "upgrade" didn't migrate a couple of accounts so I had the manually add them back. But then it would only sync the last 2 days worth of inbox from the server. Whilst I was trying to fix this mess I stumbled across this thread - before opening up terminal I followed another link in a comment by @Mr. Richard P (back on page 2), and linked off that page I found this page
https://support.microsoft.com/en-gb/office/the-new-outlook-for-mac-6283be54-e74d-434e-babb-b70cefc77439
There is a way to revert back to the legacy version, there is a link in the Help menu.
Clicking, opened a modal asking why I want to revert - so I told them, then it updated the database, quit and reopened in the legacy version.
Who knows how long this option will stick.