well, hmm, I genuinely attempted to help. "I had designated some coal to be auto mined, before i cancelled that and decided to just remove the whole layer with channeling." i would like to note this action falls afoul of rule no. 1
there could be some kind of bug related to this previous action, where the automine designation is somehow left attached to the rock invisibly. I would simply abandon the hill and look for fresh soil. if you can reproduce this event it might be worth reporting on the tracker