Backup tape protection time

Hi All,

I'm using the HDP to backup my system. I set the protection time is 3 months.

And i did backup 6 months ago. Month by month with a tape full backup.

Today, i check the tapes and some of them were expired after 6 months
There are no data in the expired tape.

I donot know why? As i know, the protection time is to make sure your data cannot be overwrite.
But why my data gone away even i did nothing?

Please someone help to answer my question, i was socked to see it.

Thank you