Car temperature sensor shows far too little, why?

Hello, I retrofitted an oil cooler with an oil pressure and oil temperature sensor with additional gauges on my Toyota. (Similar to the picture)

The oil pressure is working fine, but there might be something wrong with the temperature. It shows 0°C when cold, but it should be around 10°C.

And when it's warm, it goes up to about 40°. I have a thermostat in the oil cooler that opens at 80°, and the oil is cooled.

Under load it shows me 52°.

It looks to me (as stupid as it may sound) that I need to multiply the display by 2. Then the values ​​would be correct, but somehow it all seems pretty strange.

Can anyone help me with this?

PS: I also swapped the display for a different one, and it shows exactly the same values. And to test it, I connected the signal cable to ground, and it showed 120°. (So, it can only display a maximum of 120°, which is actually correct.) And yes, the sensor is brand new.