From the UK CAP 371:
"13 Maximum FDP - Aeroplanes
13.1 Standard reporting times prior to flight must be specified by an operator. Pre-flight duties are part of the FDP. A period of duty must be allowed for post-flight activities:
the minimum for major operators is 30 minutes, 15 minutes for others. If this “period” for post FDP duties is routinely exceeded then the post FDP duty period stated in the scheme must be revised to better represent the actual time taken. The
time spent between reporting for a flight and the completion of post-flight tasksdetermines the length of the subsequent rest period."
If your employer is ending your duty time while you are sitting in the cockpit post-flight, they are violating the regulations. At least in the UK.