ok that sounds like a normal operation except for the fact that it moves, so yes it sounds like the parking brake isn't working correctly.

The parking brake has to be engaged for it to start, but it also has to hold the truck (not roll), and if i'm not mistaken the transmission shouldn't engage until you release the brake.

So check the wiring at the park brake switchs and check for any issues.
There are 2 switches on the monotrol pedal trucks, the switch on the right hand side of the park brake bracket which energizes the seat warning circuit when handbrake is released, it also puts the transmission in neutral by de-energising the directional solenoids.
The switch on the left hand side of the park brake bracket prevents the engine from starting unless the park brake is applied.

F004V03171C

I dont think its a clutch pack it doesnt just jump back it continues to roll until you hit the brakes or flip the montrol to forward.

first we might need a serial number from the truck dataplate to get the correct info for this truck.

my first thoughts , if there are no errors showing on the display or MIL lights lit when this occurs then there may be a stuck control sensor switch or possibly the reverse clutch pack may be hung up so when you first start the truck it causes the truck to jump briefly until it lets go and shifts back into neutral.
And yes there might be a park brake switch issue but usually if that switch is not made the truck wont start.

There are many things that could cause this symptom but as i stated, having a serial number would be more helpful to help any of us help you figure this out. (unless someone has already encountered this problem and already knows what it is). :o)