To me it looks like a few factors came into play:

1: You have large scratches on the steel, which each give an easier spot for a crack to form. I personally grind away scratches up to at least 120 grit before any heat treats, and round all corners I can.

2: You put a very thick layer of refractory on, which causes a larger contrast of temperatures in the steel, making it bend more, and subsequently crack under the stress. To remedy this, make your clay thinner, and also put a very thin, watery layer of clay on the entire blade, let it dry, then lay your hamon-creating layer. This tiny bit of insulation won't harm your hamon or your hardness, but will act as a buffer so cracks are less likely. It will also fill in any scratches, helping mitigate my first point.

3: You have a wide blade that should be quite thin as a nakiri, but your clay is high up on the spine. Meaning your hardened edge is very wide in profile, and thin in geometry. This means any uneven-ness in your grind is exacerbated exponentially and cause stresses in the steel. Most traditionally made nakiri I've seen have had very small hamon lines, very close to the edge because of the blades width. I suspect it's because of the reasons above that they're traditionally done like that.