It's fairly normal. With undamped derailleurs, thoroughly preventing it would require very high chain tension, which would compromise shift quality and increase the drivetrain friction on the lower run of the chain. Bikes ridden off-pavement see a lot more big impacts, which is why their derailleurs have included dampers on the cage pivot. This usually takes the form of a friction plate that prevents the cage from rotating very freely in the counter-clockwise direction (when viewed from the bike's drive side), attached to a spring clutch that "disconnects" the friction plate when the cage is rotating clockwise and wrapping up loose chain. For road usage, it's usually viewed as a minor issue not worth bothering with.
Oftentimes road bikes come with a piece of transparent tape over the drive-side chainstay to protect it. A few older steel touring bikes actually had braze-ons to suspend a piece of material above the chainstay, and many older bikes achieved modest protection by simply routing the shift cable above the chainstay rather than below or internally. A few companies make chainstay protectors that wrap around the chainstay, and you can get similar results by just wrapping the chainstays with something like cloth tape.
A chain being too long can exacerbate the issue, because it will cause the rear derailleur's cage spring to be in a more relaxed position that applies less tension to the drivetrain, and because the chain has a tiny bit more free mass to get tossed around.
These two reasons are also why shifting to larger sprockets generally reduces slap. On my gravel bike (which does not have a damped derailleur), I'll often get into the big chainring on rough descents even if I'm not pedaling, because it stabilizes the chain. It increases the chain tension, and keeps more of the chain on the sprockets rather than floating around in space.
Last edited by HTupolev; 06-14-20 at 03:32 PM.