Interesting that there was a crash -- the builder does a pre-scan check to clear every single block in the space he's about to build, so that's why he "didn't do anything" then appeared to randomly start. It's a bit more obvious if you don't place in a field that he's scanning and breaking. He starts from the sky first and then to the ground. One thing I could do is that he skips smartly on blocks in the sky when breaking.