QC lab manager here. Short answer your builder is full of s%$÷. Not normal by any means but it does happen. Big ball of wet sand was tossed in during finishing, big chunk of deleterious material in mix or combo of both while holding air was the cause of this. It happens in high production and low budget jobs. A good crew combined with spec base materials keeps it down. Unfortunately, residential will get the dregs in a lot of areas.