I have problem querying table with variable in IN function.

SELECT 
    s.date,
    (SELECT
         GROUP_CONCAT(value) 
         FROM value 
         WHERE id_value 
         IN(s.ref_values)
    ) 
    AS vals
    FROM stats s
    ORDER BY s.date DESC
    LIMIT 1

Where s.ref_values is '12,22,54,15'. I get only one return for first number (12).

When I insert that value directly in IN(12,22,54,15) it finds all 4.

So, there must be problem with using variable in IN. What am I doing wrong?