I am experiencing the unnerving problem of having some of the messages in my inbox just disappear. 

It seems to happen to messages that have been in there for longer than a certain amount of time (not sure how long exactly). They are usually messages that I have flagged and know I need to act on, but have not gotten around to doing so yet.  I leave them in my inbox so I will see them and be reminded that I still need to do something about them, but at least twice now I have opened my inbox and found everything deleted.

I tried searching through all flagged mail, thinking I could find them that way but could not.  I then tried checking the trash to see if they had somehow gotten moved into the Trash or Spam folder, but again, they were not there.

This has never happened to anything that I have marked as Archive, or have moved to a special folder - only to things that I have left in my inbox for several weeks, even though they were flagged.

I am dealing with the issue now by just making sure that I archive, or move to a folder, everything that I want to keep, but I would like to be able to leave some things in the inbox without having to worry about them disappearing.