With that lever valve controlling normal and bypass there will be another hidden valve. There will be a one way check valve on the output port of the water heater. It’s purpose is to allow hot water to flow out of the tank into the hot water line but to not allow water to flow from the hot water line into to tank. You need this when winterizing with the lever valve in the bypass position.
I do not know if this valve has failed. It will be hidden on the back of the water heater tank and it does not need human intervention (when working properly). But if it has failed shut it would create the symptoms you are describing.
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