The circuit you are showing implies bottom plate sampling. With bottom plate sampling you care about the charge in the capacitor, not about the voltages at A and B. Voltages at A and B will be changing when you turn off M2 because of it's signal dependent charge injection. But the charge in the capacitor should stay pretty much constant because the right side of C doesn't have any resistive path to ground. Does it do that in your simulation?
BTW, for bottom plate sampling you don't really have to turn M1 way before M2. You just need to turn it off slightly before M2, say 100-200-300ps.