Mail threads have random chronology

I'm running Mail 12.4, Mojave 10.14.4, Mac mini. In Mail, many threads are suddenly showing their messages in random chronological order. I'm not aware of having changed anything that might cause this.

To clrify the problem: I don't mean the "most recent messages at the top" thing, I mean that a highlighted thread will have, for example, the messages in the panel on the right going (top to bottom) May 11, June 5, June 27, May 11, July 10, June 27, etc. Random chronology (and timings).

I started a post in Apple's forum, and they had me run Mail in a test Mac account, which did not show the problem. They then said I should check Mail's prefs and rules, but I have my doubts that this is relevant.

Can anyone here suggest what could be wrong?

 

Can anyone offer any suggestions about what might be going on here, please?

 

Thanks MoBoMac, but I have checked prefs, including view. As I said, this is NOT about "most recent at the top" etc, but the appearance of a random order.