I'm also seeing this. Very similar set up MS365 with OAUTH. Works fine for a few days, then stops. Last time this happened I set logging going. Basically we're seeing one occurrence of the following two log entries for every cron run.
API ERROR (403) Ticket Denied
and
Ticket denied Ticket rejected (spammer@spamdomain.com) by filter "SYSTEM BAN LIST"
It looks like the cron job encounters this message and then stops processing any further messages.
Manually checking the mailbox today and we see the offending banned message, followed by many newer, unread legitimate emails. Manually deleting the banned message gets things moving again.
Hope this helps?
Chris