I am not sure about your model. Many have ways of throttling the water flow so as to keep the temperature at the desired flow temperature, and they will simply throttle the flow to maintain temperature. My system has a simple keypad that I can turn the heating off with and still leave the system plugged in.
The tanks can be an area of bacteria growth, while not common, it is possible. Can you isloate those temporarily and retest? What is the maximum temperature you can set your units too? Mine max out at 120 degrees, but can be "safety bypassed". If you are maintaining a temperature below 140, bacterial growth can definetly occur in the plumbing system. Since most tankless designs are set at 120 or less, this can be more of a problem. You could see if your system temperature can be increased to 140, run it through all of the hot water plumbing at that elevated temperature for a couple hours to kill off any bacteria, and see if that helps. Tankless heaters and bacterial control issues is still young in its research. The addition of small tanks for recirc make this an eare of concern. Please keep us updated.
Use extreme caution above 120 degrees, extreme scalding potential! Do not let kids or the elderly near water that has that high of a temperature. Immediately reset the water temperature to a safe temperature when you are done.