[SQL] DISTINCT ON needing Bitmap Heap Scan

I have a query like:

SELECT
DISTINCT ON (services.group_id)
services.service_id,
...
FROM services
WHERE services.group_id IN ('67240181b97477f054f9b1bc', '67240181b97477f054f9b1be')


I have an index on group_id

Why does Postgres need to do an extra heap scan? I just want any 1st match with a matching group_id. Notice it should alr have the correct records from the Bitmap Index Scan which already checks for matching group_id

Unique (cost=609.73..611.66 rows=50 width=240) (actual time=0.353..0.385 rows=2 loops=1)
-> Sort (cost=609.73..610.70 rows=386 width=240) (actual time=0.353..0.364 rows=386 loops=1)
Sort Key: group_id
Sort Method: quicksort Memory: 133kB
-> Bitmap Heap Scan on services (cost=11.56..593.15 rows=386 width=240) (actual time=0.030..0.180 rows=386 loops=1)
Recheck Cond: ((group_id)::text = ANY ('{67240181b97477f054f9b1bc,67240181b97477f054f9b1be}'::text[]))
Heap Blocks: exact=39
-> Bitmap Index Scan on ix_services_group_id (cost=0.00..11.46 rows=386 width=0) (actual time=0.019..0.020 rows=386 loops=1)
Index Cond: ((group_id)::text = ANY ('{67240181b97477f054f9b1bc,67240181b97477f054f9b1be}'::text[]))
Planning Time: 0.505 ms
Execution Time: 0.434 ms

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