I don't believe this is a Kinsai mutation. Look at the leaves. They are all very very narrow. The white with red tips mutation of 'Kinsai' is 'Togenkyo'. There might be a pink version of 'Kinsai', but I can't track it down. And if there would be one, it would need an additional mutation to make the leaves this narrow .
There is a mutation that causes both leaves and petals to be very narrow. It is found in the old cultivar 'Seigaiha'. But recently, breeders have created new varieties, including 'Starstyle Pink'. Now, this surely isn't 'Starstyle Pink' either. But among satsuki, there is 'Otakumi', which this isn't either. So I don't know what it is, but it might be a hybrid of 'Seigaha'. Nuccio bros also made a few similar to this. But those are unlikely to turn up in Belgium. Did it come from Japan or Asia?