We have what might be a similar issue. I've been able to prove that mail flow between internal users is passed as expected UNLESS they are using the Outlook mobile app, which mysteriously applies mail flow rules as if the sender is someone outside the domain.
Sender@abc.com to Recipient@abc.com with HTML attachment using Outlook desktop client: OK
Sender@abc.com to Recipient@abc.com with HTML attachment using Outlook mobile app: Blocked by exchange\mail flow\rules attachment rule.
Maybe this is a temporary bug? You would think authenticated exchange users would be treated as internal senders regardless of client used.