ORDER BY DESC is ignored in STRING_AGG function when DISTINCT is also specified

select 
	string_agg(col1, ',' ORDER BY col1 DESC) as orderByDesc, 
	string_agg(col1, ',' ORDER BY col1 ASC) as orderByAsc, 
	string_agg(DISTINCT col1, ',' ORDER BY col1 DESC) as distinctOrderByDesc, 
	string_agg(DISTINCT col1, ',' ORDER BY col1 ASC) as distinctOrderByAsc 
from (
	SELECT 'b' as col1
	UNION ALL
	SELECT 'c' as col1
	UNION ALL
	SELECT 'a' as col1
        UNION ALL
	SELECT 'c' as col1
) x