I am having the same issue. I found a long post on the apple board with hundreds of customers with the same issues with their microsoft accounts in the mail application. It is unclear if its a microsoft issue or an apple issue at this point. I have tried removing and re-adding the accounts but it does not work.
One work around mentioned on the apple boards is to install the microsoft edge app and log into it, it creates a link between your device and microsoft, but for me that only corrected the issue on one of my accounts.
This issue is only happening on my apple devices, my pc and laptop seem to be fine, but my imac, macbook, iphone and ipad all have the issue.