Any difference in chain slap between a 11t sprocket and a 12t sprocket which is a whopping 1/6" taller is going to be marginal. Nothing related to the change in cassette is material in any way, and the OP can rest comfortably that his bike is fine as it.

BTW- since the cassette and chain were replaced at the same time, any change would be more likely the result of a longer chain, than a smaller cassette. However the chain is within guidelines, and need not be shortened, except if the OP wants to save the weight of 1" of chain.

To the OP ----- YOUR BIKE IS FINE AS IS. Stop obsessing over stuff that doesn't matter, and ride it. You can prevent chain slap by pedaling over bumps which will keep the upper loop taught. Or you can add a padded chain protector (as do many mtn bikers) which reduces the sound of slap. A cheap padded protector can be masd of clear PVC tubing, split up it's length and kept in place with wire ties.