A G Suite account was the first thing that I thought of as well, but no, this is straight gmail.com. I actually tested with my personal G Suite account, and I got similar results. Because I'm in the support team for the manufacturer, we've been hearing reports from a large number of customers who have been complaining of this over the past 48 hours.

I don't see any indication of that from the simple output provided from the message trace.

Also, it only seems to affect messages sent from Gmail via SMTP. Messages sent from the web interface are delivered properly.

That's a great suggestion, but the message gets tossed by the M365 spam filters before it reaches the inbox and I never have a chance to see the headers. All I'm able to see is the snippet of information that doesn't show anything actionable.