Different TTL behavior with unique checks in PRIMARY KEY vs. secondary UNIQUE KEY

With the following table:

create table t (a int primary key, ts bigint unsigned not null, b int unique key) engine=rocksdb comment "ttl_duration=3600;ttl_col=ts;";

MyRocks disallows duplicate PK values to be inserted with read filtering disabled, and allows conflicts with filtered rows with read filtering enabled:

mysql> insert into t values (1, 0, 1);
Query OK, 1 row affected (0.00 sec)

mysql> set global rocksdb_enable_ttl_read_filtering=0;
Query OK, 0 rows affected (0.00 sec)

mysql> insert into t values (1, 0, 2);
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'

mysql> select * from t;
+----+---++------
| a | ts | b |
+----+---++------
| 1 | 0 | 1 |
+----+---++------
1 row in set (0.00 sec)

mysql> set global rocksdb_enable_ttl_read_filtering=1;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from t;
Empty set (0.00 sec)

mysql> insert into t values (1, 0, 2);
Query OK, 1 row affected (0.00 sec)

mysql> select * from t;
Empty set (0.00 sec)

However, unique keys conflicts with filtered rows occur no matter if rows filtering is enabled or not:

mysql> set global rocksdb_enable_ttl_read_filtering=0;
Query OK, 0 rows affected (0.01 sec)

mysql> select * from t;
+---+----+------+
| a | ts | b |
+---+----+------+
| 1 | 0 | 2 |
+---+----+------+
1 row in set (0.00 sec)

mysql> insert into t values (2, 0, 1);
ERROR 1062 (23000): Duplicate entry '1' for key 'b'

mysql> set global rocksdb_enable_ttl_read_filtering=1;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from t;
Empty set (0.00 sec)

mysql> insert into t values (2, 0, 1);
ERROR 1062 (23000): Duplicate entry '1' for key 'b'

If the above behavior is expected, I've been unable to find any traces of it in the docs.