The bacon naturally retreats from the heat as it shrinks, sort of like plastic wrap to a flame.
I've found a couple ways to prevent that, the easiest is to use a meat weight. If you were to go to a restaurant supply, they are flat metal plates with a wooden/plastic handle straight up.
The weight will also slightly decrease cook time, because it's channeling the heat through the top and straight through the meat. The bacon comes out nice and flat though.