You misunderstood. You need to check the job logs- click 'expand all' then click 'find' search for "Media Operation" .
Verify that each job log indicates an append operation. One or more of you logs will probably indicate "Overwrite" and that's what we need to troubleshoot.
What is the rotation period? How many days from when the tapes are removed from the library elapse before they get reinserted.
I would like to recommend changing your overwrite protection from 'None' to a more specific time period, so that you don't have to erase the tapes (which are overwritable anyways based on your settings) and prevent accidental overwriting of data.