ALTER TABLE ... ENGINE = MERGE is binlogged w/o ddl tag

In binlog the GTID event for

ALTER TABLE t3 ENGINE = MERGE UNION (t1,t2)

must have DDL flag, that is mysqlbinlog output would be something like

#260121 19:51:18 server id 1  end_log_pos 371 CRC32 0x1d25b159  GTID 0-1-1 ddl

...

ALTER TABLE t3 ENGINE = MERGE UNION (t1,t2)}}

for the above ALTER group of events.
This fact is found out by knielsen in MDEV-21107.

The lack of the DDL flag indeed can cause the MDEV-21107 description assert, and its presence rules out possibilities
of such execution paths.