Apple Mail not synching "read" status between two MacBook Pro's

Situation:

MacBook Pro 15" Intel (MBP 15) and MacBook Pro 16" M1 (MBP 16) both running

1. macOS 15.7.1
2. Apple Mail to connect to my email host, who's running Microsoft 365
3. Both machines are situated on my desk and Apple Mail is running
4. In a browser window I have opened Outlook in order to view the email on the host.

Problem:

I open an unread email on the MBP 15, read it, and its status changes to "read" on that machine; however, it's still "unread" on the MBP 16 as well as in Outlook on the email host. The other way around, i.e., opening a new email on the MBP 16 results in the expected behavior of the Apple Mail on the MBP 15: that email is also displayed as "read". As far as I can tell, the email account in question is configured exactly the same on both machines. I'm at a loss to pinpoint where the problem even occurs.

I don't recall experiencing this problem when the account in question wasn't an Exchange account, but one that used IMAP and SMTP.

Any suggestions?