Inconsistent behavior upon adding foreign key to RocksDB table

install soname 'ha_rocksdb';

create table t (a int) engine=RocksDB;

--error ER_NOT_SUPPORTED_YET

alter table t add foreign key(a) references x(x);

alter table t drop foreign key xx, add foreign key(a) references x(x);

MariaDB [test]> alter table t add foreign key(a) references x(x);

ERROR 1235 (42000): This version of MariaDB doesn't yet support 'FOREIGN KEY for the RocksDB storage engine'

MariaDB [test]> alter table t drop foreign key xx, add foreign key(a) references x(x);

Query OK, 0 rows affected (0.342 sec)              

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> show create table t \G

*************************** 1. row ***************************

       Table: t

Create Table: CREATE TABLE `t` (

  `a` int(11) DEFAULT NULL,

  KEY `a` (`a`)

) ENGINE=ROCKSDB DEFAULT CHARSET=latin1

1 row in set (0.004 sec)