mariabackup --lock-ddl-per-table hangs in FLUSH TABLES due to MDL conflict if ALTER TABLE issued

mariabackup --lock-ddl-per-table  --backup --user user --password password --stream=xbstream --parallel=4 --tmpdir=/tmp | gzip > backup.xbs.gz

alter table testpk add column MARIA_ID bigint unsigned not null auto_increment primary key

MariaDB [(none)]> show processlist;

+----+-------------+-----------+------+---------+------+---------------------------------+--------------------------------------------------------------------------------------------+----------+

| Id | User        | Host      | db   | Command | Time | State                           | Info                                                                                       | Progress |

+----+-------------+-----------+------+---------+------+---------------------------------+--------------------------------------------------------------------------------------------+----------+

|  1 | system user |           | NULL | Daemon  | NULL | InnoDB purge worker             | NULL                                                                                       |    0.000 |

|  2 | system user |           | NULL | Daemon  | NULL | InnoDB purge worker             | NULL                                                                                       |    0.000 |

|  3 | system user |           | NULL | Daemon  | NULL | InnoDB purge worker             | NULL                                                                                       |    0.000 |

|  4 | system user |           | NULL | Daemon  | NULL | InnoDB purge coordinator        | NULL                                                                                       |    0.000 |

|  5 | system user |           | NULL | Daemon  | NULL | InnoDB shutdown handler         | NULL                                                                                       |    0.000 |

| 63 | root        | localhost | rick | Query   |  113 | Waiting for table metadata lock | alter table testpk add column MARIA_ID bigint unsigned not null auto_increment primary key |    0.000 |

| 80 | root        | localhost | NULL | Query   |   52 | Waiting for table flush         | FLUSH NO_WRITE_TO_BINLOG TABLES                                                            |    0.000 |

| 81 | root        | localhost | NULL | Sleep   |  117 |                                 | NULL                                                                                       |    0.000 |

| 82 | root        | localhost | NULL | Query   |    0 | init                            | show processlist                                                                           |    0.000 |

+----+-------------+-----------+------+---------+------+---------------------------------+--------------------------------------------------------------------------------------------+----------+

9 rows in set (0.00 sec)

     10 libaio::??(libaio.so.1),LinuxAIOHandler::collect(os0file.cc:1893),LinuxAIOHandler::poll(os0file.cc:2038),os_aio_linux_handler(os0file.cc:2092),os_aio_handler(os0file.cc:2092),fil_aio_wait(fil0fil.cc:5330),io_handler_thread(srv0start.cc:343),start_thread,clone

      3 pthread_cond_wait,wait(os0event.cc:166),wait_low(os0event.cc:166),os_event_wait_low(os0event.cc:166),srv_resume_thread(srv0srv.cc:935),srv_worker_thread(srv0srv.cc:935),start_thread,clone

      1 pthread_cond_wait,wait(os0event.cc:166),wait_low(os0event.cc:166),os_event_wait_low(os0event.cc:166),srv_resume_thread(srv0srv.cc:935),srv_purge_coordinator_suspend(srv0srv.cc:935),srv_purge_coordinator_thread(srv0srv.cc:935),start_thread,clone

      1 pthread_cond_wait,wait(os0event.cc:166),wait_low(os0event.cc:166),os_event_wait_low(os0event.cc:166),buf_resize_thread(buf0buf.cc:3083),start_thread,clone

      1 pthread_cond_wait,wait(os0event.cc:166),wait_low(os0event.cc:166),os_event_wait_low(os0event.cc:166),buf_dump_thread(buf0dump.cc:777),start_thread,clone

      1 pthread_cond_wait,inline_mysql_cond_wait(mysql_thread.h:1149),inline_mysql_cond_wait(mysql_thread.h:1149),thd_destructor_proxy(mysql_thread.h:1149),start_thread,clone

      1 pthread_cond_wait,inline_mysql_cond_wait(mysql_thread.h:1149),inline_mysql_cond_wait(mysql_thread.h:1149),handle_slave_background(mysql_thread.h:1149),start_thread,clone

      1 pthread_cond_timedwait,os_event::timed_wait(os0event.cc:286),os_event::wait_time_low(os0event.cc:413),srv_monitor_thread(srv0srv.cc:1754),start_thread,clone

      1 pthread_cond_timedwait,os_event::timed_wait(os0event.cc:286),os_event::wait_time_low(os0event.cc:413),srv_error_monitor_thread(srv0srv.cc:1923),start_thread,clone

      1 pthread_cond_timedwait,os_event::timed_wait(os0event.cc:286),os_event::wait_time_low(os0event.cc:413),pc_sleep_if_needed(buf0flu.cc:2718),buf_flush_page_cleaner_coordinator(buf0flu.cc:2718),start_thread,clone

      1 pthread_cond_timedwait,os_event::timed_wait(os0event.cc:286),os_event::wait_time_low(os0event.cc:413),lock_wait_timeout_thread(lock0wait.cc:547),start_thread,clone

      1 pthread_cond_timedwait,os_event::timed_wait(os0event.cc:286),os_event::wait_time_low(os0event.cc:413),ib_wqueue_timedwait(ut0wqueue.cc:163),fts_optimize_thread(fts0opt.cc:3032),start_thread,clone

      1 pthread_cond_timedwait,os_event::timed_wait(os0event.cc:286),os_event::wait_time_low(os0event.cc:413),dict_stats_thread(dict0stats_bg.cc:451),start_thread,clone

      1 pthread_cond_timedwait,inline_mysql_cond_timedwait(mysql_thread.h:1186),timer_handler(mysql_thread.h:1186),start_thread,clone

      1 pthread_cond_timedwait,inline_mysql_cond_timedwait(mysql_thread.h:1186),my_service_thread_sleep(mysql_thread.h:1186),ma_checkpoint_background(ma_checkpoint.c:709),start_thread,clone

      1 pthread_cond_timedwait,inline_mysql_cond_timedwait(mysql_thread.h:1186),MDL_wait::timed_wait(mysql_thread.h:1186),TABLE_SHARE::wait_for_old_version(table.cc:4410),close_cached_tables(sql_base.cc:454),reload_acl_and_cache(sql_reload.cc:334),mysql_execute_command(sql_parse.cc:5384),mysql_parse(sql_parse.cc:7902),dispatch_command(sql_parse.cc:1806),do_command(sql_parse.cc:1360),do_handle_one_connection(sql_connect.cc:1335),handle_one_connection(sql_connect.cc:1241),start_thread,clone

      1 pthread_cond_timedwait,inline_mysql_cond_timedwait(mysql_thread.h:1186),MDL_wait::timed_wait(mysql_thread.h:1186),MDL_context::acquire_lock(mdl.cc:2138),MDL_context::upgrade_shared_lock(mdl.cc:2333),mysql_inplace_alter_table(sql_table.cc:7245),mysql_alter_table(sql_table.cc:7245),Sql_cmd_alter_table::execute(sql_alter.cc:324),mysql_execute_command(sql_parse.cc:6208),mysql_parse(sql_parse.cc:7902),dispatch_command(sql_parse.cc:1806),do_command(sql_parse.cc:1360),do_handle_one_connection(sql_connect.cc:1335),handle_one_connection(sql_connect.cc:1241),start_thread,clone

      1 pthread_cond_timedwait,inline_mysql_cond_timedwait(mysql_thread.h:1186),inline_mysql_cond_timedwait(mysql_thread.h:1186),cache_thread(mysql_thread.h:1186),one_thread_per_connection_end(mysql_thread.h:1186),do_handle_one_connection(sql_connect.cc:1354),handle_one_connection(sql_connect.cc:1241),start_thread,clone

      1 __poll,vio_io_wait(viosocket.c:945),vio_socket_io_wait(viosocket.c:108),vio_read(viosocket.c:184),my_real_read(net_serv.cc:892),my_net_read_packet_reallen(net_serv.cc:1162),my_net_read_packet(net_serv.cc:1146),do_command(sql_parse.cc:1242),do_handle_one_connection(sql_connect.cc:1335),handle_one_connection(sql_connect.cc:1241),start_thread,clone

      1 __poll,handle_connections_sockets(mysqld.cc:6584),mysqld_main(mysqld.cc:6050),__libc_start_main,_start

      1 nanosleep,os_thread_sleep(os0thread.cc:230),srv_master_sleep(srv0srv.cc:2412),srv_master_thread(srv0srv.cc:2412),start_thread,clone

      1 nanosleep,os_thread_sleep(os0thread.cc:230),btr_defragment_thread(btr0defragment.cc:772),start_thread,clone

      1 do_sigwait,__sigwait,signal_hand(mysqld.cc:3505),start_thread,clone