Could be belt slip, or could be the spool or the Pulleys slpping form the shaft.

Its comon for this car. Dial ur brakes out a little or use drag brakes.

You can look at it as a protection to not destroy ur entrire drive train by these parts slipping, in races you dont do full stop race anyway.

And its not healthy to do dead stop liek this anyway for an RC car regardless on shaft or belt.