This may or not be applicable to the truck you are working on.
The Nissan truck I had to work recently had an issue of going to neutral and losing mast hydraulic functions shortly after start up.
As this truck was sent to us from a distant facility I have no familiarity with who usually works on it and the cunning of their local operators, but I am always suspicious that someone may have "messed with" seat switch wiring (because I have seen many attempts to defeat the OPSS on a few trucks).
But back to the recent Nissan Platinum truck. I checked the OPSS switch and wiring on the hood and all was well, no signs of tampering.
Next I checked the weather pack electrical connector (near the right rear FOPS pillar) that joins the OPSS hood harness to the chassis harness. Upon separating the halves of that connector a bit of green powder fell into my hand. I extracted the individual terminals from the connector halves for a thorough cleaning and "tightening" and reassembled them with some "no corrode compound" and that took care of that problem.
But the truck was not showing any code because of the problem.
The trouble your truck has may indeed be something amiss with the FNR switch or its wiring.
I would think that if it actually is confined to the FNR circuits the mast hydraulics would continue to operate.