Do you have the account enabled on another device that might be downloading and then deleting the messages? If so check the settings to see if you are using POP3 instead of IMAP.
I do have another account enabled using IMAP. It is set up to delete trash on exit. However this particular email was not in that trash. I found it in recovered trash in the Comcast account and tried to move it to Inbox. That's when it disappeared. Did notice that if I had put it in one of my other folders I could have retrieved it.
When you move an email from Recovered Deleted Items, or from Trash, it will put it back in chronological order. If it was a few days or a week old, did you scroll down to where it belongs?
I figured it out! If you move a recovered email from your trash, even if you select "Inbox", it won't go there. It goes to the last folder you had moved something to, as that folder remains highlighted. Believe that's true for moving any email -- you just can't get it back in your inbox, but you can get it to one of your folders. What I did was make a folder for recovered emails.
Found that you need to make sure INBOX is properly highlighted when you want to move an email back into inbox. The shaded line is already sitting on Inbox when you're preparing to move an email. That doesn't mean it has been selected. Make sure you click on it. It will brighten slightly.