That's actually by design, 4 March 2017 LDU: https://youtu.be/mtjVLI0wEy8
If I'm not mistaken, 100 is 4x required cooling, 50 is 1x required cooling (dead on, as you say), and 0 is 0.5x required cooling; any less and the engine is too under cooled / too unreliable (same as in the Kee version).
And the reason grilles aren't needed, is because no other fixtures are needed, so why should cooling be needed as well? So cooling was/is decoupled from fixtures. People who don't want to design car looks don't have to in anyway now, (except perhaps aero fixtures, though that should probably be changed too?).
The only thing I'd suggest is change the slider text to make it in line with what it does; 50 isn't 0.5x cooling after all, and the scale isn't linear, so it isn't intuitive at a glance without tool tips.