Hi Robert,
1) Extra CRLF character in the end sometimes gets translated to a NULL ( which is a valid value for masking), try re -uploading lookup file with no empty line at end and in UTF-8 encoding with no special characters.
2) Since masking will convert a value to another value in lookup it is possible that a value gets masked to itself, try sorting the SL lookup list.
NULLS, Blanks, empties are valid values in masking and are distinctly preserved, white-spaces are treated as word strings.
--Hims