ORIGINAL: yallaair
...The reason is that in the center of the high pitch prop, angle of attack is above 18º. hence, that disc area is stalled static.
Yallaair,
Once a prop, any prop, has formed a vortex, the air flowing through it is at roughly pitch speed and thus the effective AoA of the prop's blades is close to 0º, i.e.
not stalled.
Before a vortex is formed, or if it cannot be formed; like if the prop is in ground effect, most of the blade will be stalled and more of it, if the prop's pitch is higher.