Pistons are acting weird in my basalt generator

These two basalt generators are identical, but only the one on the right has a piston that works properly. While the right piston fires with every basalt block forming, the left piston fires continuously, creating a redstone clock. I think this is a bug, but I could be mistaken due to my lack of redstone know-how. If this were to be fixed/explained, I would be very grateful.