I have a late 2012 iMac, a mid-2013 MacBook Air, and a mid-2015 MacBook Pro, all running Mojave.
This issue only occurs on the iMac: I'm looking at a list of email messages in the inbox. If I hit the 'delete' key to delete an email message out of my inbox list, and then change my mind, I can do a command-z to undelete and the message reappears in the list (as it should). But if I try to open the message it's just a blank window, no content. But if I quit out of Mail.app and then reopen, I can then click on the message to read it. Seems like a weird caching problem.
This does not happen with the MacBooks, and it never occurred on any of the machines when I was running El Capitan.
Anyone have any ideas what's causing this?