Area in world where parts of world disappear

I created a world in Minecraft 1.13.2 (seed 890940218721954). In this world, there are colored squares on the ocean surface (at -795 78 1611). When the player flies over these squares, parts of the world disappear (see attached video).

I tried to upload the world file, but at 20 MB it's too large. You can download it here: http://www.kooistra.de/zeug/B&M%20Welt.zip