Custom shuffle function stopped working in 9.6

Good evening,

after switching to 9.6.2 from 9.5.3 the following custom function has
stopped working:

CREATE OR REPLACE FUNCTION words_shuffle(in_array varchar[])
RETURNS varchar[] AS
$func$
SELECT array_agg(letters.x) FROM
(SELECT UNNEST(in_array) x ORDER BY RANDOM()) letters;
$func$ LANGUAGE sql STABLE;

In 9.5.3 it was shuffling characters:

words=> select words_shuffle(ARRAY['a','b','c','d','e','f']);
words_shuffle
---------------
{c,d,b,a,e,f}
(1 row)

But in 9.6.2 it has stopped doing so:
words=> select words_shuffle(ARRAY['a','b','c','d','e','f']);
words_shuffle
---------------
{a,b,c,d,e,f}
(1 row)

Any suggestions for a better shuffling function please?

Regards
Alex