Protection is not set on the tapes, it is set on the session in the IDB. As long as the session contained on these media is protected, the tape will not be written over, but, once protection disappears, the tape is fair game
Every day at noon, a Maintenance procedure runs, which looks for media where protection has expired. If it finds any, the catalog in the iDB is deleted to save on disk space. At this point, you can no longer do a restore from the media for this session (although a Restore using List from Media is still possible)
When you export a tape, you are actually deleting the Data Catalog Binary Files that holds all information about what was backed up onto that tape. Tis is why, after exporting a media, the slot will show as Data Protector Foreign, that is, DP is aware that teh media is in DP format, but is no longer in the IDB
Then, when you import media, new Data Catalog Binary files are created, and you can now restore ... until 12 noon, where you will lose your catalog again
When importing media and when the Import finishes, you shoud IMMEDIATELY go to the new session ID, right click on it, and pick Change Data Protection, and set it to some value, I usually recommend Permanent. It can always be recycled later