If your heater valve is stuck open you have an extra radiator running that is in the loop and cooling the engine even when tstat is closed. Logic dictates that will make the engine run a little cooler than the set point so your symptoms seem to match the conditions described.

You can use a hose pinch clamp to close off the heater core to confirm before you actually replace the heater valves.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)