Citroën C5 X7 1.6 DV6C Is the thermostat working properly?

[Palex12]

Hi everyone,
I'm looking for some advice on my Citroën C5 X7 with the 1.6L (DV6C) engine in cold weather conditions. I've attached a graph of my coolant temperature readings. At idle, the engine never seems to reach the thermostat’s opening temperature. It only gets there when the engine is under load, for example during normal driving. However, once I'm on the motorway and let off the gas to coast, the temperature starts to drop again.
These measurements were taken at about -6°C with winter covers (wind deflectors) in place. Is this typical behavior for the DV6C in cold climates, or should I suspect a failing thermostat?
Thank you in advance for your help, and I wish everyone a very Merry Christmas and happy holidays!
Best regards, Pauli