Presume you've unplugged and re-plugged all the relevant connectors in case of a dodgy contact? .... and did you replace the voltage-regulator (if your car uses one (my car is old-school so I've been caught out before commenting on modern models 🙂)
As the IR tells you nothing is amiss beyond the gauge I'd check that the gauge rises 'normally' from cold and let it sit til the temp rises and triggers the fan... then watch for it to fall back as they run.
At that point I'd put a spot of paint on the glass to mark where the new 'normal' was and drive it until Winter ... but then, I'm a barbarian!
K
