Take some pictures of the piping going into the unit. What is the two way valve set for? Should be set to external. Sounds like you probably have either a crossover return system or a dedicated system. If the Navien check valve is not the source of the crossover the next likely suspect is a single handle shower cartridge or an external check valve. I have seen a 2 handle utility sink faucet where the handles are left open and there's a shut off on the spout which lets water mix. You can probably rule the Navien out by shutting the outlet valve on the heater and see if you still have the same flow on the hot side as the previous test.
