Entire index scanned, but only when in SQL function?

Hi there,
I’m on Postgres 13.11 and I'm seeing a situation where an INSERT...SELECT statement seq scans an index, but only when wrapped in a SQL function. When invoked directly (via psql) or when called via a PL/pgSQL function, it only reads the index tuples it needs, resulting in much better performance. I can solve my problem by writing the function in PL/pgSQL, but I'm curious why the pure SQL version behaves the way it does.

Here's my table --

\d documents
+-------------------+------------------+----------------------------------------+
| Column | Type | Modifiers |
|-------------------+------------------+----------------------------------------|
| document_id | integer | not null generated always as identity |
| product_id | integer | not null |
| units_sold | integer | not null |
| sale_date | date | not null |
... some other columns ...
+-------------------+------------------+----------------------------------------+

CREATE INDEX idx_philip_tmp on documents (document_id, product_id);

Here's the SQL function which will use that index --

CREATE OR REPLACE FUNCTION fn_create_tasks(product_ids int[])
RETURNS void
AS $$
-- Create processing tasks for documents related to these products
INSERT INTO
processing_queue (document_id)
SELECT
DISTINCT document_id
FROM
documents
JOIN unnest(product_ids::int[]) AS product_id USING (product_id)
;

$$ LANGUAGE sql VOLATILE PARALLEL SAFE;

96498 is a product_id that has one associated document_id. When I copy/paste this statement into psql, it executes quickly, and pg_stat_user_indexes.idx_tup_read reports 2 tuples read for the index.

INSERT INTO
processing_queue (document_id)
SELECT
DISTINCT document_id
FROM
documents
JOIN unnest(ARRAY[96498]::int[]) AS product_id USING (product_id)
;

When I copy/paste this into psql, I expect it to perform just as quickly but it does not. pg_stat_user_indexes.idx_tup_read reports 64313783 tuples read (which is the entire index).

SELECT fn_create_tasks(ARRAY[96498]::int[])

If I rewrite fn_create_tasks() in PL/pgSQL, it behaves as I expect (executes quickly, pg_stat_user_indexes.idx_tup_read = 2).

SELECT fn_create_tasks_plpgsql(ARRAY[96498]::int[])

My rule of thumb is that SQL functions always perform as well as or better than a PL/pgSQL equivalent, but this is a case where that's not true. If anyone can give me some clues as to what's happening here, I'd appreciate it.

Thanks
Philip