Usually when this happens it’s a failing thermostat. Mine did exactly this. The temp gauge got up to it’s usual place just about but the engine was running a full 15 degrees [C] cooler than it should. The temp gauge in the mark 3 [as with oil pressure “gauge”] cannot be relied upon to do anything useful, I now have a proper separate temp gauge which shows that the standard temp gauge responds way too early and shows fully warm before the car is anywhere near it. The same happens when the car gets hot too, the standard gauge sits happily at the normal mark whilst the engine is actually nudging 100 degrees. By the time the standard gauge has recorded any significant rise in temp you’ve already cooked the motor.
A simple OBD2 reader attached to your phone will tell you what temp the engine is really running at.