I have experienced this and I can recreate this reliably.
Make sure you have chocks on and parking brake on
Initiate GSX pushback
When GSX ask you to release parking brakes, remove chocks, the aircraft slide forward
When you release parking brake, the aircraft stop sliding forward.
A temporary workaround that worked for me is:
It doesn't matter if you have chocks on or off
Initiate GSX pushback
When GSX asks you to release the parking brakes, release the parking brakes first. If you have chocks on, make sure the chocks are the last thing you remove.
Viola, no sliding forward