Although I have no experience with that model, it is my understanding tankless water heaters will reduce water flow to maintain the output temperature setpoint as temperature rise is inversely proportional to flow rate. If you turn off the heater and there is no flow/pressure drop it would appear this is what is happening. If this did not happen earlier in the life of your water heater, it would appear there is a problem related either to the control of the flow reduction valve, or the input of heat to the water passing through the heat exchanger. You mention use of a water softener. A softener, correctly implemented and programmed (a bit of an assumption, not everybody gets this right) should guarantee the heat exchanger is not coated with calcium carbonate. However, internal scaling of the heat exchanger would certainly be one way the heat input to water flowing through the exchanger could be reduced. A correctly-working tankless heater would then maintain correct water temperature in this impaired state by reducing the water flow to compensate.