Chain slap is very common. The small sprockets have the chain passing pretty low over the chainstay, so it doesn't take much for slap to happen. The only thing that prevents it upper loop chain tension when you pedal. However when you coast, the tension provided by the RD cage spring isn't enough to prevent slap, which is by design because more spring tension would increase power loss.

Don't worry about preventing slap, instead focus on managing it. I use a length of split vinyl tubing strapped to the chainstay at three points with tie wraps. It's not fancy but works well, not only protecting the chainstay (who cares anyway) but muffling the noise of impact very well.