Index on nested JSONB field is not used - sql

Question

I created an index on a nested JSONB field:
CREATE INDEX foo_idx ON some_table(cast(content->'meta'->>'version' AS int));
but the select query still does a full table scan:
select *
from some_table
where (content->'meta'->>'version')::INT <= 9000
LIMIT 1;
I also tried to express the query like:
select *
from some_table
where cast(content->'meta'->>'version' AS INT) <= 9000
LIMIT 1;
with the same result.
Query plan:
Limit (cost=0.00..1.06 rows=10 width=52)
-> Seq Scan on some_table (cost=0.00..38429.27 rows=361441 width=52)
Filter: ((((content -> 'meta'::text) ->> 'version'::text))::integer <= 9000)
What do I miss here?
Edit: It was more a coincident. I added a ORDER BY random() to the query and got the following query plan:
Limit (cost=31644.83..31644.83 rows=1 width=52) (actual time=0.017..0.017 rows=0 loops=1)
-> Sort (cost=31644.83..32548.43 rows=361441 width=52) (actual time=0.016..0.016 rows=0 loops=1)
Sort Key: (random())
Sort Method: quicksort Memory: 25kB
-> Bitmap Heap Scan on game_object_user (cost=6769.60..29837.62 rows=361441 width=52) (actual time=0.011..0.011 rows=0 loops=1)
Recheck Cond: ((((content -> 'meta'::text) ->> 'version'::text))::integer < 9000)
-> Bitmap Index Scan on foo_idx (cost=0.00..6679.23 rows=361441 width=0) (actual time=0.009..0.009 rows=0 loops=1)
Index Cond: ((((content -> 'meta'::text) ->> 'version'::text))::integer < 90000)
Planning time: 0.074 ms
Execution time: 0.040 ms
The index was used.