Hi,
so maybe that is the reason why the tape initially became poor. Maybe due to polling or RSM the header became overwritten when the DP backup still was acive. So if after a scan the media still is blank, this means the header information is not available and without media header the import becomes impossible. But the behaviour was not initated by DP. Anyway and sorry, it seems you will not be able to restore data from this specific media anymore.
Best regards
Daniel