The bug

The graphical world loading progress is shifted for some superflat worlds. The center around which the chunks are loaded is not in the middle of the square.
Probably related to this is that the screen displays "100%" for a while despite still loading chunks.

How to reproduce

1. Create a superflat world with the following preset

   minecraft:ender_chest;127;decoration
   

2. Watch the loading progress
   → The center of the chunks appears to be offset from the middle of the screen