Bug with subqueries in recursive CTEs?

I played with a silly example and got a result that surprises me:

  WITH RECURSIVE fib AS (
        SELECT n, "fibₙ"
        FROM (VALUES (1, 1::bigint), (2, 1)) AS f(n,"fibₙ")
     UNION ALL
        SELECT max(n) + 1,
               sum("fibₙ")::bigint
        FROM (SELECT n, "fibₙ"
              FROM fib
              ORDER BY n DESC
              LIMIT 2) AS tail
        HAVING max(n) < 10
  )
  SELECT * FROM fib;

   n  | fibₙ 
  ----+------
    1 |    1
    2 |    1
    3 |    2
    4 |    2
    5 |    2
    6 |    2
    7 |    2
    8 |    2
    9 |    2
   10 |    2
  (10 rows)

I would have expected either the Fibonacci sequence or

  ERROR:  aggregate functions are not allowed in a recursive query's recursive term

Yours,
Laurenz Albe