All read emails disappeared from all folders. Repairing any folder brings them back but they disappear again if I click on another folder

TB 60.7.2 (64-bit) on 2015 Macbook. All read emails disappeared from all folders. Unread emails are visible. If I repair any folder the missing emails appear. But when I look in another folder and come back, they have disappeared again. Tried creating a new folder. Then repaired a folder. Moved the emails to the new folder. Went to a different folder and returned to the new folder. The emails had disappeared again. I'm thoroughly puzzled. Help anybody? Thanks.