The [break] marker gets added when you use these. If you repeat Export and Import a couple of times, you'll see plenty of [break] added at the end of each row (last cell in a row).
I'm using windows 10. And after some digging it seems that, StringBuilder class in this environment encodes new lines as \r\n, but when you import the file you cut the lines based on \n, which gets you a lot of leftover \r, which when exporting gets turned into a bunch of [break]'s. And after a couple of repeats a bunch of extra line breaks appear.
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