I created table with following structure and inserted some test data.
Empty parts(rows = 0) still exists even if all rows from the table were deleted over TTL.

create table temp_ttl_partition_test
(
rec_date DateTime,
user_id String
)
engine = MergeTree
partition by formatDateTime(rec_date, '%Y%m%d%H')
ORDER BY user_id
TTL rec_date + interval 10 minute
SETTINGS ttl_only_drop_parts=1;

CH version = 20.5.2.7

Expected behavior: TTL_ONLY_DROP_PARTS must drop empty parts