Index of acl rules might not be used properly

MariaDB [nextcloud]> explain SELECT DISTINCT b.id FROM oc_deck_boards b LEFT JOIN oc_deck_board_acl acl ON b.id = acl.board_id WHERE (owner = 'admin') OR ((acl.participant = 'admin') AND (acl.type = 0)) OR ((acl.participant IN ('group', 'group2')) AND (acl.type = 1));
+------+-------------+-------+-------+------------------------------------------+---------------------+---------+----------------+------+------------------------------------+
| id   | select_type | table | type  | possible_keys                            | key                 | key_len | ref            | rows | Extra                              |
+------+-------------+-------+-------+------------------------------------------+---------------------+---------+----------------+------+------------------------------------+
|    1 | SIMPLE      | b     | index | idx_owner_modified                       | idx_owner_modified  | 263     | NULL           | 1    | Using index; Using temporary       |
|    1 | SIMPLE      | acl   | ref   | deck_board_acl_uq_i,deck_board_acl_idx_i | deck_board_acl_uq_i | 8       | nextcloud.b.id | 1    | Using where; Using index; Distinct |
+------+-------------+-------+-------+------------------------------------------+---------------------+---------+----------------+------+------------------------------------+
2 rows in set (0.003 sec)

MariaDB [nextcloud]> show create table oc_deck_board_acl;
+-------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table             | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
+-------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| oc_deck_board_acl | CREATE TABLE `oc_deck_board_acl` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `board_id` bigint(20) NOT NULL,
  `type` int(11) NOT NULL,
  `participant` varchar(64) NOT NULL,
  `permission_edit` tinyint(1) DEFAULT 0,
  `permission_share` tinyint(1) DEFAULT 0,
  `permission_manage` tinyint(1) DEFAULT 0,
  PRIMARY KEY (`id`),
  UNIQUE KEY `deck_board_acl_uq_i` (`board_id`,`type`,`participant`),
  KEY `deck_board_acl_idx_i` (`board_id`),
  KEY `idx_participant_type` (`participant`,`type`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+-------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.004 sec)

Start by reproducing the reported EXPLAIN query against oc_deck_boards and oc_deck_board_acl, then inspect the CREATE TABLE output and the listed indexes, especially idx_participant_type. Compare the optimizer's chosen index with the ACL predicates and determine what behavior should be considered correct; done means the query uses an appropriate index or the concern is documented as expected behavior.