The game only renders a square of 17x17 grids around the grid the player is in (with 10x10 tiles per grid). Trying to spawn zombies outside of that square will fail (in code getGridSquare(x, y, z) will return nil). Depending on the randomly chosen angle, the distance to the edge of the spawn zone can vary between 90 and sqrt(2) * 90 ~ 128 tiles. If your Minimum Spawn Distance is above 128 then the zombies WILL NOT SPAWN, period. If it's within a 90-128 range, they may or may not spawn, depending on the random angle, but I wouldn't test your luck on this.

I took me one anti-climactic evening of disappointment and debugging to figure this out, so don't repeat my mistake: set the spawn distance somewhere between 70 and 90 or leave them as defaults, and also keep the noise pulse range above the maximum spawn distance, so that the zombies actually come towards you after spawning.