sub-select in IN clause results in sequential scan

Postgres consistently does a sequential scan on the child partitions
for this query

select * from partitioned_table
where partitioned_column > current_timestamp - interval 8 days
where x in (select yy from z where colname like 'aaa%')

If I replace the query with

select * from partitioned_table
where partitioned_column > current_timestamp - interval 8 days
where x in (hardcode_value)

The results are in line with expectation (very fast and uses a Bitmap
Index Scan on the column X)