We had a very similar problem when we upgraded from 10d to 11d. When we exported a tape - even one we had just completed a successful backup to - then imported the same tape again, it would be listed as Unknown Media and be placed in the Scratch media set.
Ultimately, we moved all of the Unknown Media from the Media tab into the Retired Media folder (on the tree on the left of the media tab) and deleted the Unknown media from there. Since then, everything has been working much better. I can now export and import a tape confident that it will return with the correct information. I don't have any more Unknown Media.
I hope this helps. Good luck!
Sam