I fired up a new world, excited to try out biome blend. So I flew around and saw this. It's an entirely new world. It also creates tiny lag spikes when breaking or placing blocks in that square. Also when I make a hole and try to enter it, it doens't let me. Generating the world again with the same seed doesn't generate the giant square again.
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Please only put one bug per report. This makes sorting and managing them much easier.
As for the stated title and screenshots, it is a duplicate of MC-125807. Please use the search function to see if the bug has already been reported.