Thanks for uploading your collection. I was able to reproduce the issue! This looks like a bug.
It happens when the sort field is set to something other than the first field. It’s set to English in your case.
@dae, It seems different parts of the code have different assumptions about whether to check the first field or the sort field. Checking the first field is the expected behavior according to the manual.
Both of note_is_duplicate_or_empty and showDupes work with the first field, but write_dupes works with the sort field. I assume write_dupes is in the wrong here?