As I understand, the engine starts and stops when the fuel is cut off.
Is it right (please adi m. confirm)

Timing setting,
I simply try to figure if everything is OK with the engine itself.
By timing I mean ignition timing.
Are the spark plugs OK?

I am still not convinced if it's not an LPG system problem.
If the mixer, for example, is dirty and the piston inside "hangs" it may cause similar outcome as you described.

The same with the coil. There were some very tricky issues with the ignition coils on 39X trucks.
The older (black) ones were behaving strange. It could happen, that on one truck it was OK, on another it was faulty.
Please confirm if you have the blue ignition coil.

Check if the wiring is OK. Perhaps you have some short circuit.

I will consult the issue with one of my specialists who serviced tens of such trucks. Maybe he finds something.