Has happened a few times now, line up and start thundering down the runway. Some distance down the runway the rudder will go to neutral position and stop responding to joystick inputs. If you're lucky and near/past VR you can pull back and use ailerons to correct the issue. Eventually the rudder comes back into operation.

My theory is the hydraulic system. Using brakes or tiller/rudder will deplete the hydraulic pressure very quickly. The plane has 4 systems and when you dab the brakes you can see a noticeable drop in system pressure. Usually A and B, then they are slow to recover.

I let the FE sort the panel out but from past experience I turn on the PTU B-A system ON as A system will deplete quickly without the help, you will also get warning messages flash up briefly about system pressure. Should he put B2 and C2 to AUTO instead of leaving them in the off position?

Above 60 kts, tiller and nosewheel operation inhibited, which is most likely what you're experiencing.

Any rudder axis input above 60 kts will only move the rudder. Tiller (Nosewheel Steering) Axis will not be operational.

In v1.0.7 there is an Flight Model update coming to give more effectiveness to rudder alone but the rest appears to be by design and not a result of any hydraulics.

Please verify against the 60kts threshold for what you're experiencing.