artwork inside a symbol randomly gets deleted during save??

So I have a problem where artwork inside a symbol will disapear.   

Example:

I have a character.  I animate the scene, save etc.   Close the file.   Next time I open it up, the eyes are missing.  But you can see a white square where the symbol is.  In fact, the symbol is still there.  When you go inside the symbol however, all that's inside is a single blank frame.  No matter how many layers or frames was inside it before, there's just a single blank frame.

This doesn't happen often, but when it does, it can set me back an hour or two fixing things, depending on the symbol that it happens to.   Currently, it's a set of symbols in a prop that have emptied.   

Any ideas?