I really, really wish Apple would resolve this. If it is not the cause, then something else out of user control has to be the problem. If that truly is what is going on, it is ridiculously hypocritical, petty, and insulting. Somebody once told me that Apple pays Microsoft licensing fees for using Exchange on iOS, but does not pay the fees on Mac OS. Work arounds are inconsistent such that they seem to work only by coincidence. You can search the forums here and find issues dating back to Catalina. Deleting and reinstalling my Exchange account takes about 3 days to synch and only temporarily solves the problem. I have over 350k messages in archives mostly on my mac and some in the cloud on office 365. For me, it has been going on for years, dating back to when I had my own Exchange server. The only thing that is unreliable is an exchange account in Mail on Mac OS. Web Outlook (which I despise even more) works fine.

Using Office 365, and Outlook for Mac (which I despise) works fine. Deleting Mail's saved application state worked for a while a couple of times on Mojave. None of the suggestions work permanently.