I don't know if this explains it, but old Yanmars had that lower hose with a drain cock that seemed to be designed to assure the block got entirely drained when the radiator was drained for the winter. Ditch water is cheaper than year-round antifreeze! Back when the owner manual described this, and running without a battery. Both 'features' aimed toward the rice paddy farmer who used the tractor for rototilling only a few days per year. (The tiller was in some cases semi-permanently mounted, not 3-point.) Thermostat? No thermostat, in my ancient Yanmar.
So you might be seeing a 'feature' that's a holdover from an ancestor model.