Have you looked at the Exchange Online and Defender portals for clues? Especially in the Message Trace section of Exchange Online?
I recently helped troubleshoot a similar issue - we were suddenly getting emails disappearing when sending to M365 customers. No spam or quarantine, just disappearing down a black hole like you described. We sent a test message to a M365 customer who could help run the message trace, and we discovered that the SVG logo in our email signature was being flagged as a phishing attack. We had been using this logo for about a year without any issues, but suddenly Microsoft just decided to block it without warning.
I recently helped troubleshoot a similar issue - we were suddenly getting emails disappearing when sending to M365 customers. No spam or quarantine, just disappearing down a black hole like you described. We sent a test message to a M365 customer who could help run the message trace, and we discovered that the SVG logo in our email signature was being flagged as a phishing attack. We had been using this logo for about a year without any issues, but suddenly Microsoft just decided to block it without warning.