My Liebherr skills are quite rusty these days, but one of the things to check is make sure your park lever is not drifting down while operating. I have seen this before. There should be an adjustment for how stiff the lever operates.
Another possibility is the steering pedals. If there is dirt stuck in them, or they have a rusty hinge mechanism, they cannot return to their at rest position, or if the sensors on those pedals have come out of adjustment or are failing, then that can cause intermittent transmission disengagement.
Also, a faulty speed sensing circuit can cause intermittent transmission disengagement as well.
Unfortunately, the best way to check and adjust those pedals is with the Pedal Test/Adjustment box which only the dealer would have (or should have). They should also have the CEP12 software for pulling codes as well as the diagnostic test box (different than the pedal box) for troubleshooting the transmission circuit (which would include the speed sensors).