I have a similar issue with an operator presence seat sensor switch on my 2020 Yanmar SA424. In the warmer months it seems to work as would be expected most of the time. If the parking brake is set it will start and continue to run when the operator gets off the tractor. Sometimes in the warmer months the engine will continue to run even when I forget to set the parking brake and exit the tractor.
In the winter I seem to encounter the greatest challenges. (This is my second winter with it.) I keep it in an unheated shed, and with the parking brake set it will not remain running when I get off to allow it to warm up. I have also experienced a few no crank situations in the coldest months. Bounce on the seat a couple of times and the engine cranks and fires up, stand up and the engine shuts down. All while the parking brake is set. Plowing snow and set the parking brake to dismount and change the blade angle, and most times, the engine shuts down.
It is intermittent, I am very close to disabling it. Dealer mechanic was out last fall to service another piece of equipment and I asked him to review this unit while he was here. As would be expected, it worked every time as intended. So there was nothing he could do to "fix" the issue for me. (Unit still under manufacturer warranty.)
Since the parking brake has an adjustable sensor we did adjust it to ensure it always engages correctly when the parking brake is set. On occasion this was the issue for the no crank situation I had experienced shortly after delivery. Had to push down on that pedal till the shank flexed before the switch would engage and allow the engine to crank. Fairly certain that this condition has been corrected now.
I too have 50 & 60 year old tractors where memory muscle and operator awareness are the safety devices. However, I also have a young great nephew who wants to help me with the lawn mowing this coming summer, and while he needs to learn, I would like to minimize the protentional for a preventable injury while he is doing so. His "worldly" experiences have been very limited to date. Hence I am close, but have not disabled the sensor, as then I would need to do as others have suggested and remember to enable it before someone else is in the operator seat.