I think you're on the money with the top hose I didn't even think of it. Perhaps it has something to do with the thermostat coil being fed through.
In the thermo fan kit did they give you a rubber fillet. The rubber fillet must be inserted between the top tank hose spigot and the thermocouple tube as you install the top hose otherwise you can experience coolant leakage from the top hose around the thermocouple tube.