Inconsistent results given by array_agg vs group_by

[jadami10]

This is a simplified version of a user query

SELECT 
  start_month,
  end_month,
  array_agg(segment_category, 'STRING', true) AS segment_category,
  distinctcount(segment_category) AS distinctcount
FROM metrics_table
WHERE subset_name = 'All Data'
  AND start_month = '2024-01-01'
  AND end_month = '2024-09-01'
GROUP BY 1, 2
UNION ALL
SELECT 
  'no group by' AS start_month,
  'no group by' AS end_month,
  array_agg(segment_category, 'STRING', true) AS segment_category,
  distinctcount(segment_category) AS distinctcount
FROM metrics_table
WHERE subset_name = 'All Data'
  AND start_month = '2024-01-01'
  AND end_month = '2024-09-01'

What we see here is

start_month	end_month	segment_category	distinctcount
2024-01-01	2024-09-01	segment_a,segment_b,segment_c,segment_d,segment_e,segment_f,segment_g,segment_h	8
no group by	no group by	segment_a,segment_b,segment_c,segment_d,segment_f,segment_g,segment_h	8

without the group by, we still get the right distinct count, but one of the groups is missing in array_agg. This happens consistently.