[Question] What are these detached chunks in the nether?

[SevenOutman]

Hi, first of all thanks for creating this wonderful tool!

I've just learned how to use this tool generally, but got this question. When I loaded my nether, besides the main, continuous map, there appeared several "fly lands" - chunks that are completedly surrounded by black boxes and detached from the main map. When I add a overlay of inhabited time, they all appear to be 0.

What are they?

Here are the screenshots.