The following test case for the database dbt3_s001 from innodb_ext_key.test
returns the execution plan with REF access method though EQ_REF is expected.

create table t0 (a int);
insert into t0 values (1), (2), (3), (4), (5);
create index i_p_size on part(p_size);
set optimizer_switch='extended_keys=on';
explain
select * from t0, part ignore index (primary)
where p_partkey=t0.a and p_size=1;

MariaDB [dbt3_s001]> explain select * from t0, part ignore index (primary) where p_partkey=t0.a and p_size=1\G

• • • • • • • • • • • • • • • • • • • • • • • • • • • 1. row ***************************
                                                      id: 1
                                                      select_type: SIMPLE
                                                      table: t0
                                                      type: ALL
                                                      possible_keys: NULL
                                                      key: NULL
                                                      key_len: NULL
                                                      ref: NULL
                                                      rows: 5
                                                      Extra: Using where
                                                    • 2. row ***************************
                                                      id: 1
                                                      select_type: SIMPLE
                                                      table: part
                                                      type: ref
                                                      possible_keys: i_p_size
                                                      key: i_p_size
                                                      key_len: 9
                                                      ref: const,dbt3_s001.t0.a
                                                      rows: 1
                                                      Extra:
                                                      2 rows in set (0.00 sec)