Hi Pedro Rocha
In regards to Tape Library Compression, if the setting in the Data Path Properties is set to use "Hardware Compression", what that means is that we will leverage the Tape LIbrary's native capability to compress the data.
Documentation - https://documentation.commvault.com/commvault/v11/article?p=12329.htm
So once CommVault hands off the I/Os to the Tape Library, it is up to the responsibility of the Tape Library to do the compression.
One area that you can check, is to confirm whether the licensing for "Compression" is enabled on the Tape Library. Certain Tape Library Vendor will require additional licenses enabled on the Tape Library (i.e. compression)
Hopefully the above information helps