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The problem is that author_sort is empty, which is arguably appropriate when there is no author. You can see this by sorting on author -- the "Unknown"s will show up at the beginning of the list unless author_sort has been manually set. Note that the same thing will happen if you clear author_sort by hand.
To avoid this problem, set the author sort for all books where it is empty. Search for author_sort:false, select the books, open bulk metadata edit, check the "Automatically set author sort", and press OK. Alternatively, set it to what you want, which may be something other than 'Unknown'.
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