O índice não é usado ao fazer uma consulta com PostgreSQL CTE

Dado que uso a seguinte tabela:

CREATE TABLE operations (
    asset_id varchar(255) NOT NULL,
    event_id varchar(255) NULL,
    updated_at timestamptz NOT NULL DEFAULT CURRENT_TIMESTAMP
);

e o seguinte índice:

CREATE INDEX idx
ON operations ( event_id);

Quando executo a seguinte consulta

WITH ROWS AS (
SELECT
    *,
    ROW_NUMBER() OVER (PARTITION BY asset_id ORDER BY updated_at DESC) AS row_num
FROM
    operations
)
SELECT
    *
FROM
    ROWS
WHERE
    row_num = 1
    AND event_id IN ('bb87d265b3c82098f7cd6913a4eb6df2', '4780d32075328c6f6f0ebe4e7fd48662')
    AND operation != 'DELETE';

o índice não é usado:

Subquery Scan on rows  (cost=7159.77..7909.77 rows=1 width=598) (actual time=48.159..48.160 rows=0 loops=1)
  Filter: (((rows.event_id)::text = ANY ('{bb87d265b3c82098f7cd6913a4eb6df3,4780d32075328c6f6f0ebe4e7fd48662}'::text[])) AND ((rows.operation)::text <> 'DELETE'::text) AND (rows.row_num = 1))
  Rows Removed by Filter: 20000
  ->  WindowAgg  (cost=7159.77..7559.77 rows=20000 width=598) (actual time=38.352..46.491 rows=20000 loops=1)
        Run Condition: (row_number() OVER (?) <= 1)
        ->  Sort  (cost=7159.77..7209.77 rows=20000 width=590) (actual time=38.343..39.519 rows=20000 loops=1)
              Sort Key: operations.asset_id, operations.updated_at DESC
              Sort Method: quicksort  Memory: 2911kB
              ->  Seq Scan on operations  (cost=0.00..467.00 rows=20000 width=590) (actual time=0.012..2.993 rows=20000 loops=1)
Planning Time: 0.144 ms
Execution Time: 48.436 ms

Se eu remover a condição row_num, o índice será usado. por exemplo :

WITH ROWS AS (
SELECT
    *
FROM
    operations
)
SELECT
    *
FROM
    ROWS
WHERE
    event_id IN ('bb87d265b3c82098f7cd6913a4eb6df3', '4780d32075328c6f6f0ebe4e7fd48662')
    AND operation != 'DELETE';\

Alguém pode explicar por que a primeira consulta não usa o índice?