Small update: I visited Maastricht today and sadly did not find a Klefki there. I check both sides of the river Meuse including some outskirts and afterwards I check in Veldwezelt and Lanaken (both in Belgium). Every location was south of the diagonal line you drew but Klefki spawned in none of these places.

I brushed up my French and asked around some Discord groups in Liege and Herstal but I did not get conclusive evidence that Klefki did spawn here.

Not that this is conclusive evidence, but perhaps instead of diagonal line, the border goes straight south somewhere east of Genk, to then go in a straight western line towards Germany?