Wren core transform SQL failed UNION ALL expected 2 inputs, but found 4

[grieve54706]

WITH vals AS
  (
    SELECT 1 x, 'a' y UNION ALL
    SELECT 1 x, 'b' y UNION ALL
    SELECT 2 x, 'a' y UNION ALL
    SELECT 2 x, 'c' y
  )
SELECT x, ARRAY_AGG(y) as array_agg
FROM vals
GROUP BY x;

DataFusion error: This feature is not implemented: UNION ALL expected 2 inputs, but found 4

BigQuery function reference
https://cloud.google.com/bigquery/docs/reference/standard-sql/aggregate_functions#array_agg