This has me worried. I have about the same frequency and duration of wear, but mine look fine, for now at least. It's always wet/rainy here and 90% of my shoes have leather soles. I have overshoes, but they would get destroyed riding my motorcycle. Wonder how soon I'll need some resoles.
That "crack" could easily be a cut from glass or a sharp rock. My brown 1Ks had this problem after one of my hikes in scree. This alone is a good reason to add some type of half sole to the boot.