Mondeo mk4 Engine cooling weird issue

I've a mondeo mk4 2008 1.8 TDCI 120 PS for 5 month now and i'm having a weird issue. The analog engine gauge go to halfway (so presumably 90 degres C) but with the diagnostic mode on the car I can see "Engine temp" digitally only going to 75 degrees C .. The weird part is that when i'm using engine brake to slow down ,my analog temp gauge going down to the 1/4 (so approx 75 degrees C) and the  "Engine temp " digital on diagnostic mode going down to 66 degrees C.

If i let the car idle the gauge goes after 15-20 Minutes to the halfway on engine gauge so 90 degrees again, and the digital Engine temp via diagnostic mode goes to 82-85 degrees C.. (which is the only thing it seems ok in my issue) but after driving less than 3 minutes, it goes back to 75 Degrees C on digital diagnostic Engine temp and the analog gauge stays at the middle so 90degrees ..

75 degrees C seems low for me even for a diesel car .. especially with outside temp in summer like now

I've already changed the thermostat but the problem is still happening, the temps is so erratic,

So for me the issue is that running engine 75 degrees seems low and it get even lower down to 65 degrees when using engine brake

Do you think that changing the sensor would fix the issue ? Or do you have any other fix in mind ?

by the way here’s a little video of the issue (sorry filmed in French as it’s my main language)

thanks !