Mac Mail emails disappearing while deleting others

I have several Mac Mail accounts. One of them, an IMAP account, has been doing a bizarre thing for several iOS versions now and it is continuing with High Sierra. As I'm going through and deleting emails, an email I haven't deleted will disappear. It's usually the third in the sequence. For example, if I delete two emails in a row, the third one disappears. I delete the next two after that, the third one will disappear again. When I quit out of Mail and launch it again, the disappeared emails return. Of course, that's a hassle to have to do. Anyone else having this problem or know of any solution? It's very aggravating. I'm on a late 2015 iMac 27-inch retina, if that matters. With a Benq 27" side monitor also running.