Mining the ground is raising it beneath me

Been working on a canal for about 8 hours or so now. Got everything connected all the way to the center, and had to clear out a hill to do it. I noticed about halfway through that some of the ground I'm levelling with the hoe isn't dropping down to the level of my feet so I start smacking it with my pickaxe to get it beneath sea level.

To my surprise the ground popped up underneath me the first time I struck it and remained at that height every subsequent time. I figured you'd also like to know that it continued giving me stone as well. So not only is this preventing me from building the canal I was working on (I'm gonna see if I can just go around) but it looks to be an infinite stone exploit as well.