This is only a guess, but what I think you are running into is a conflict with an existing WP_Query parameter. “year” is an argument that already has a special meaning to WP_Query. Are the posts being ordered by the post date “Year”? Try changing your clause name to something else. It is generally a good idea to use things like “year_clause” and “num_clause” to make sure you don’t end up with conflicts in naming.
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