What happens if you try to copy a message from the troubled folder to a different folder? Does the message show up with it's contents in the different folder?

The folder is displayed as empty so there's nothing there to copy. If I drag a message into it it disappears too.

When I look at folder properties it shows 4453 messages, up from 4445 before. I assume the increase is from filter rules.

Also if I right click on the folder, select Search Messages, and search for something I know is there it works: I see a list of matching messages. If I double click on a message it's displayed correctly.

I fixed it. I made a new folder, searched for all messages in the bad folder with length > 0, moved those messages to the new folder, adjusted the filter to go to the new folder, deleted the bad folder, and renamed the new folder to the bad folder name.

The fix in the previous post did work. However the problem just recurred with a different folder.

Exact same symptoms. A folder that had 5083 messages displayed only 6 messages from 7/14 on. The exact repair method I described on 7/8 worked. I may have made some filter changes around that time to filters that moved messages to this folder to now move and mark them as read but I'm not sure. Otherwise I have no idea what's triggering this.

I'm glad I know how to repair this but have no idea how to make it stop happening. Any ideas?