jsonb subscript operator returns null when key is fetched from table.

PostgreSQL 15.1 on x86_64-pc-linux-musl, compiled by gcc (Alpine 12.2.1_git20220924-r4) 12.2.1 20220924, 64-bit

WITH cte AS MATERIALIZED (SELECT 'foo' k, '{"foo":"bar"}'::jsonb j)

SELECT j[k], j[k || ''] FROM cte;

-- returns

-- null, "bar"

-- expected

-- "bar", "bar"