Thank god I found this thread.  This is entirely not documented clearly (the disappearing files attach-once feature).  Why have a bloody DOC CENTER if it isn't for managing documents you can re-use! We have a significant and critical "Terms and Conditions" doc we want to attach to every invoice (and really QB should allow for a "attach this to every invoice" feature) but it is SO tedious to have to reattach.. . if the Doc Center worked as "appearances suggest it should" i would be absolutely ideal. 

By the way the lousy "oh your folder was renamed" directions that you find when you search for this problem is a massive misdirection and waste of time.  I suspect 98% of people searching and complaining about not finding documents are becuase of this 'feature / functionality / working as designed' and almost NONE of them are because they renamed the folder, yet thats what they always get told.