For replicas that don't support GTIDs, a MariaDB primary will convert its GTID event into a Query log event with the query BEGIN. This event writes zeroes for the Query log event's thread_id, however, since MDEV-7850, where GTID events were extended with thread_id, we can now populate the thread_id of the generated Query log event with that same value. Note this GTID to Query log event logic happens in function Query_log_event::begin_event (in sql/log_event.cc).

Note that the inverse (i.e. MySQL Master -> MariaDB Slave (w/ MDEV-7850)) will correctly populate the thread_id of the GTID event using the thread_id of the Query log event, so nothing needs to be done in this case.