I have had two older Kubotas. First was a B5100DT (4x4 and 10HP) and now this L185DT. Neither would shut off with turning the key off. If you take a look at the throttle, there is a small lever down on the right that you lift up on and will reduce the fuel flow until it starves the engine and it stops. That is how I stop mine. The fuel pump is mechanical, so as long as the motor is running you get fuel. If you had an electrical pump then the key could turn it off.
Jackie
PS.. I have a new wheel on the way. Finally broke down and ordered one to replace the hybrid tractor/car wheel after I put a BIG hole in the turf tire I had on it. Also ordered two new 6-14 Ag tires/tubes. I guess since the original ones lasted about 30 years with abuse then maybe I can make them last just as long. By that time I will be so old that I may not be able to even get on the tractor by then. Already have one new tire on the rear, so will be looking for another one to match it.
