If it was my bike I'd pinch off the hose to the overflow tank - thereby stopping the gravity feed back into the radiator - and then check the level in the radiator to make sure it stays totally full and isn't sucking air from a seal, gasket or hose fault. I'd do that for peace of mind and for the intellectual satisfaction of getting to the bottom of the mystery. If there's no air gap under the radiator cap then I'd be happy. As others have suggested, maybe the MIN mark is what the level should be when cold and the MAX when hot. Another consideration is the type of coolant. The specified Triumph HD4X hybrid OAT may have extra anti-foaming qualities to justify its extra price over coolants suitable for slower revving cars. HD4X is advertised as having specific anti-cavitation properties, but the main advantages claimed for it are that it is a long-life coolant that uses organic acids instead of nitrite, phosphate, borate and amine. The hybrid part of the title usually means that it still contains the traditional aluminium protector silicate. The type/brand of coolant being used hasn't been mentioned so far, nor at what temperature gauge indication fan operation starts. Even without a fault in the cooling system, at higher revs cavitation can occur on the back of the water pump impeller blades, resulting in vapour/foaming as well as erosion of the blades. Did I mention that I prefer air-cooled bikes?