I have the union atlas, large, size 10.5 vans boots, toe is rock solid, but you have to dial it in correctly from the start.
Adjust the heelcup accordingly, it should help. For instance, I have mine at "1" to center my boot, and it fits snug and I never have to worry about my bindings.
I pushed the heelcup forward and moved the binding itself back and its solid now. Never really thought about the heelcup.