Email messages disappear from Mail on iMac after opening

Email Messages Disappear

I have had an issue for about a week where email messages are disappearing after opening on my iMac ( 27-inch 2017 iMac running Ventura [13.7.8] ), as if deleted. There have been no changes to system settings or mail settings and mail delivery was rock-solid before this time frame. This does not happen on my 2020 M1 MacBook Air (Tahoe 26.1), nor on my iPhone 16, nor on a web log-in from the iMac. This is a Gmail hosted IMAP mail account. Messages that have disappeared on the iMac all are still in the in-box on the other 3 mail views. Preview pane is NOT open on any of the devices. I’ve checked a variety of settings and find nothing amiss and found no solution in others questions in the community.

Any thoughts, ideas, comments ?