You need to specify the window frame clause:

SELECT
  id,
  value,
  FIRST_VALUE(value IGNORE NULLS) OVER (ids) AS first_valid_value,
  LAST_VALUE(value IGNORE NULLS) OVER (ids) AS last_valid_value,
  update_ts
FROM sample_data
WINDOW ids AS (PARTITION BY id ORDER BY update_ts DESC ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING)

Output: