Mail groups certain unrelated messages—why?

My wife and I are quite puzzled over this strange behavior in Mail on her iPad. (iPad Air 2, iOS 13.3) She regularly gets email from two people—let's call them Jodi and Judy. If both Jodi and Judy send email messages about the same time on the same day (this happens more often than you might imagine), Mail combines their messages in a common thread. In the Inbox sidebar, she can click the little >> to let the messages display separately. However, she can't delete one without also deleting the other! Here's a screen shot—

Jodi and Judy don't even know each other. They have completely separate email addresses; they use different ISPs; they live in separate counties. What the heck is going on here? Is there a setting we can adjust to keep this from happening ... ?