Yes, your tape really is full.
LTO8 holds about 12TB of data uncompressed, if the data is perfectly compressible then it holds about 30TB.
So 22TB sounds about right - not all data is compressible, and not all data that is compressible, compresses the same amount.
Ultimately however, NetBackup has no understanding of tape capacity, if left alone, it would try and write to the same tape 'forever'. It is the tape drive firmware that decides when a tape is full, as it reads the 'this tape is almost full' marker that is written on the tape during manufacturing. It then tells the tape driver not to send any more data and it is the tape driver that tells NetBackup to mark the tape as full.
A tape should always be able to store the native amount of data (incompressible), 12TB in the case of LTO8, so if for example the tape was being marked full at only 8TB, we can be certain this would be incorrect, although as explained, the issue would be caused by either the tape drive firmware or the tape driver,
If you truly believe the data is 'compressible' enough that more than 22TB should fit on the tape you would have to discuss it with the hardware vendor, but it would be very difficult to prove, as you would have to show that the tape drive is not compressing the data fully, so you just have to make a judgement call sometimes. I guess you could try duplicating the tape to another using a different drive and see if the tape shows the same amount of data.
Ultimately, if you did have an issue and that is caused by the tape firmware, you would also have to test using drives at a different firmware version.
Regarding the status 96, make sure the tape is showing as the same density as the drive (eg, hcart2, hcart3 etc) - that is probably the most common cause of status 96.
