Understanding the TempDB growth
Really, the only things that will cause your tempdb files to shrink are:


Someone used the 'SHRINKFILE` command to manually resize the files
The SQL Service was restarted (due to a reboot, or the service being restarted manually, etc) and the file reverted to it's configured "default size" (which can be found in sys.master_files - note that it's different from current / initial size reported in sys.database_files)


You can audit your system (using the default trace or the file growth reports in SSMS) to see when and why file shrink events happened.  You can find a history of reboots in the system_health XE session.

A side note about this:


  we shrinked and adjusted the file to 120 GB each thus allocating 960 GB to temp drive from that 1.6 TB and disabling autogrowth


Unless you are leaving room for tempdb's log file, you should really size the tempdb files to fill this dedicated drive.  Since you have autogrowth turned off, it's simply wasted space at this point =)