requested media id is not assigned to this host in the EMM database

What you see is normal - tapes in scratch should have no assign time (as seen above) and should not be in the media DB, hence the message you see.

When a tape is used for backup, pretty much the first thing that happens (if it is previously empty) is it is put in the media DB and given an assign time.  The media DB basically tracks numbers of images on tapes (+ other things).  When the images expire the image count drops to 0 and then the tape is deassigned from the media DB when one of the cleanup jobs run.  A command that looks in the media db is bpmedialist.

The other DB involved with tapes is the volume DB - this tracks physical aspects, such as location, if a tape appears in nbemmcmd -listmedia or the media part of the GUI or vmqery -a then it is in the volume DB and is known to NB.  A tape will always be in the volume DB unless it is deleted.

So in summary.

volume DB is physical  

media DB is 'image related' and only contains a tape when it has valid images

In summary you don't have an issue, or if you do, this isn't the cause.