NetBackup does NOT truncate the log files. MSSQL runs a backup to a virtual device (NetBackup) using the BCH parameters. If NetBackup reports the backup successful, MSSQL will check if the log can be truncated and truncate if required.
If the truncation fails or does not happen, the issue is from SQL and not from NBU..(if no error reported)
You can check the log truncation by querying the log_reuse_wait and log_reuse_wait_desc columns of the sys.databases catalog view.
Also if the truncation is done successfully ,It is normal for the SQL transaction log files to remain the same size even after they have been truncated. The truncate function only increases the free space in the log file to allow SQL to reuse that space vs creating a new log file.