those 'dummy' lights blinking? (the amber ones, not the red system lights)
such as the operator presence light and the fork icon?

well generally by default if the those lights are lit the truck should not move or lift. And as far as the whole dash blinking on and off? that is a normal response with this code and it puts the truck in limp mode till the problem is repaired, sometimes even disabled. It will not stop doing this till the component causing the error is fixed and code cleared from the ecm memory.

But if this truck is as you said, bypassed (a big no no) and the truck is running normally i would solve that issue first for no other reason than to make it safety compliant again.

legally if you work on this truck and leave it that way and anything else happens such as an accident and that can be tied back to the safety problem left unrepaired its gonna be a bad day for you.

If i find things such as this on lifts i insist they are repaired or i will not work on the truck from that point on.