Blocks respawn for round about 1 second

I like the new world editor and the preset Isle Land is the best one.
If I choose the preset and change 1 option under the customize World Settings to get more flying islands (to build a lot in the sky/ survival mode)
I change only the height stretch from 12,000 to 5 or 6.
Now I have a lot flying islands but all blocks in the game I mining respawn for round about 1 sec and after that they vansih forever and drop the loot.

Please fix it please