Weird. I've never used true/false columns but the
manual says "yes" & "no" are the value type, and "true" & "false" should show the empty ones. Maybe something changed? In any event it says
Quote:
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Yes/no custom columns are searchable. Searching for "false", "empty", or "blank" will find all books with undefined values in the column. Searching for "true" will find all books that do not have undefined values in the column. Searching for "yes" or "checked" will find all books with Yes in the column. Searching for no or unchecked will find all books with No in the column. Note that the words "yes", "no", "blank", "empty", "checked" and "unchecked" are translated; you must use the current language’s equivalent word. The words "true" and "false" and the special values "_yes" and "_no" are not translated.
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So I would suggest trying "empty" and "blank" but I have no idea if it will work.