This is where it fails (file->info)

 5680              /* If info() fails, then there's nothing else to do */                                                                                                                                                            │

│   5681              if (unlikely((info_error= file->info(HA_STATUS_VARIABLE |                                                                                                                                                         │

│   5682                                                   HA_STATUS_TIME |                                                                                                                                                             │

│   5683                                                   HA_STATUS_VARIABLE_EXTRA |                                                                                                                                                   │

│   5684                                                   HA_STATUS_AUTO)) != 0)) 

1. Culprit TL;DR
   Information_schema get_all_tables() is filling the data on the runtime, from frm files and data sent from storage engines.
   Handlerton file of the MERGE SE is obtained from MERGE but in order to read it we must open tables with open_tables() function that will call handler->file->extra(HA_EXTRA_CHILDREN_CHECK and fill handlerton file.
2. More about:
   Looking into the code of MERGE engine, merge table or parent table is tmerge while child table is t whose path is in the .MRG file of parent table, relative to datadir.

• Let's analyse this query with non-MyISAM tables

  # Default InnoDB SE

  MariaDB [test]> create table tmerge(t int) engine=merge union=(t);

  Query OK, 0 rows affected (0.002 sec)

  There will be frm file and MRG files

  root@2d5a561d9015:/var/lib/mysql/test# ls | grep tmerge

  tmerge.MRG

  tmerge.frm

• When creating the temporary MERGE table there is no FRM for temporary (it is in table cache in memory), but exists MRG files (in /tmp/
  Since it is used InnoDB type, create statement will succeed, but nothing else and this is covered in MERGE documentation, however documentation is missing part about usage of TEMPORARY tables.
  We can also check the table

  MariaDB [test]> check table t_temp_merge;

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

  | Table | Op | Msg_type | Msg_text |

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

  | test.t_temp_merge | check | Error | Table 'test.t' is differently defined or of non-MyISAM type or doesn't exist |

  | test.t_temp_merge | check | Error | Unable to open underlying table which is differently defined or of non-MyISAM type or doesn't exist |

  | test.t_temp_merge | check | error | Corrupt |

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

  3 rows in set (0.000 sec)

  The same error code we get even for tmerge (base table instead of temporary), since the children table t is created with default SE InnoDB.
  Check for MRG file for parent/MERGE temporary table:

  cat /tmp/#sql-temptable-1-3-a.MRG

  ./test/t

• Use MyISAM SE for the child table

  MariaDB [test]> create table t(t int) engine=MyISAM;

  Query OK, 0 rows affected (0.002 sec)

  MariaDB [test]> create table tmerge(t int) engine=merge union=(t);

  Query OK, 0 rows affected (0.001 sec)

  MariaDB [test]> show create table tmerge;

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

  | Table | Create Table |

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

  | tmerge | CREATE TABLE `tmerge` (

  `t` int(11) DEFAULT NULL

  ) ENGINE=MRG_MyISAM DEFAULT CHARSET=utf8mb4 UNION=(`t`) |

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

  1 row in set (0.000 sec)

  And we get the files

  root@2d5a561d9015:/var/lib/mysql/test# ls

  db.opt t.MYD t.MYI t.frm tmerge.MRG tmerge.frm

  root@2d5a561d9015:/var/lib/mysql/test# cat tmerge.MRG

  t

  When trying to use temporary table

  # MRG files exists in /tmp

  MariaDB [test]> create temporary table t_temp_merge(t int) engine=merge union=(t);

  ERROR 1050 (42S01): Table 't_temp_merge' already exists

  MariaDB [test]> drop table t_temp_merge;

  Query OK, 0 rows affected (0.000 sec)

  MariaDB [test]> create temporary table t_temp_merge(t int) engine=merge union=(t);

  Query OK, 0 rows affected (0.000 sec)

  MariaDB [test]> show create table t_temp_merge;

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

  | Table | Create Table |

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

  | t_temp_merge | CREATE TEMPORARY TABLE `t_temp_merge` (

  `t` int(11) DEFAULT NULL

  ) ENGINE=MRG_MyISAM DEFAULT CHARSET=utf8mb4 UNION=(`t`) |

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

  1 row in set (0.000 sec)

  MariaDB [test]> show tables;

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

  | Tables_in_test |

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

  | t |

  | tmerge |

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

  2 rows in set (0.000 sec)

  # Note that temporary table is not visible in show table status

  MariaDB [test]> show table status;

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

  | Name | Engine | Version | Row_format | Rows | Avg_row_length | Data_length | Max_data_length | Index_length | Data_free | Auto_increment | Create_time | Update_time | Check_time | Collation | Checksum | Create_options | Comment | Max_index_length | Temporary |

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

  | t | MyISAM | 10 | Fixed | 0 | 0 | 0 | 1970324836974591 | 1024 | 0 | NULL | 2022-04-20 09:06:18 | 2022-04-20 09:06:18 | NULL | utf8mb4_general_ci | NULL | | | 17179868160 | N |

  | tmerge | MRG_MyISAM | 10 | Fixed | 0 | 5 | 0 | 0 | 0 | 0 | NULL | NULL | NULL | NULL | utf8mb4_general_ci | NULL | | | 0 | N |

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

  2 rows in set (0.000 sec)

  Note from the code:

  If child is temporary, parent must be temporary as well. Other parent/child combinations are allowed.

• When debugging the bug in gdb, we may see how child MRG file gets created, but not frm file (exists in memory).
  Parent table is tmp table, that doesn't have any .frm file.

  $ find mysql-test/ |grep MRG

  mysql-test/var/tmp/mysqld.1/#sql-temptable-29e0-3-0.MRG

  $ cat mysql-test/var/tmp/mysqld.1/#sql-temptable-29e0-3-0.MRG

  ./test/t

  For the base table, FRM file of child table t exsists- mysql-test/var/mysqld.1/data/test/t.frm

I have provided a patch that solves this edge case.
It will be in the major branch when the preview is pushed.

monty or cvicentiu can you please check comment from Marko regarding the Merge table?

I fixed MSAN failure and pushed with PR 2685 to preview branch (fix).