Just to make term use more standard a freewheel/freehub/one way clutch "catches" when applying power and the drive "detaches" when not pedaling (coasting).

So I would describe the issue by saying that the ratchet doesn't disengage quickly, if I understand the problem correctly. This is the same issue I have with one bike when in the smaller rear cogs. The chain's leverage acting on the ratchet is less so if there's any "stiffness" in the ratchet/freewheel/freehub that tries to continue to drive the cogs. Top of chain stay chain slap is a usual result. The cause is some binding/friction/spot gabbiness of the cog carrier on it's core. Could be as simple as a spoke guard rubbing, a tight seal or as bad as poor manufacturing in the unit.

I agree that if the bike is new and bought from a LBS you should take it back and have them look at it. If the bike was sourced from a long distance/on line "dealer" than you will likely find you are on your own in doing the solutions. Either way, contacting the source is the first step. Andy