Electric Tankless Water Heater only achieves 106F. ?

This is a Stiebel Eltron 24 that uses 2 - 60 amp double pole breakers and #6 copper wire located only about 5' from 200 AMP modern Square D service panel. Total of 4 conductors and 1 ground. All 4 conductors have 121.5 v between conductor and ground and 243 V between conductors. There is no voltage drop from panel to heater....and no voltage drop at breaker or heater when unit is turned on. Input water temp is 55F. Each conductor draws 13 amps when heater is set to max 125F........and output temp achieves only 106F at 1.5 GPM. When warmer 90F water is input to unit....it still only achieves 106F.....and draws less amperage. It appears this unit is only trying to achieve 106F. I have exchanged the unit with manufacturer and the new unit does the same thing. Manufacturer says the unit I returned test OK in their lab....at 208V it makes 125F water at 3.5 GPM with 46F water input.

I'm baffled......what conditions could exist at my house to limit this unit to 106F out put? Why does the unit only draw 13 amps if it really is trying to make 125F output water.....it should be drawing close to 50 amps on each conductor? Why are both LEDs on the unit on solid.....indicating they think they can achieve the desired stting output temp. Im located in Montana at 4600 feet......would that screw up anything? All connections are tight, I replaced the breakers just in case. The output temp is measured directly at tank so there is no loss in the pipes or mixing issues. I'm baffled and the manufacturer help line.....while very helpfull is also baffled. I called an electrical engineer / master electritian who was very nice and had me verify the voltages over the phone.......he said they are fine and he sees no external reason unit should not perform. He asked me to check with Stiegel to ensure unit would work ok at 243 V.......they said 243 is fine and they have units working at 250V.
Thanks for the input guys....let me answer your questions......

No its not a flow issue.....it produces 106 F water at 1.5 GPM or 3.5 GPM.

No the input and output are not reversed....same as diagram and verified by tech support.......right is input, left is output.

As I stated...I sent back a unit that did not work at my house....and did work at the manifacturers lab.....so it would not seem to be a defect in the unit.

Does anyone have any ideas what external factors may cause this condition and ways to diagnose them?
The solution to the problem was plumbing. I reported earlier that the water temp was measured at the out put of the unit.....therefore eliminating heat loss in the plumbing and any issues with mixing valves that existed in the plumbing. However......I did not isolate the plumbing by turning off the valve below the test port.....so I was getting mixing backflow. It turns out a mixing valve at the output of my propane hot water tank was mixing the 145 F water output of this tank to an acceptable tempurature.....but was mixing the 125 F water output of the tankless heater to 106 F. Thanks for all the input.