No server-side block collision in new superflat worlds

At first game session in new superflat (after creating) world block collision boxes at specific height and upper not exist on server-side. 

Specific height: height of first not generated chunk (16x16x16). For example: classic - 16, redstone ready - 64

Effect: player can walk on this blocks (client side motion handling) but entities and items (server side motion handling) fall through this blocks.