In the traditional START SLAVE UNTIL, after the desired position is reached, the SQL thread stops, but the IO thread does not. With START SLAVE UNTIL MASTER_GTID_POS = ... both threads stop. It's apparently expected, but it needs to be documented in the KB (https://mariadb.com/kb/en/mariadb/global-transaction-id/#start-slave-until-master_gtid_posxxx), I couldn't find it there.

More generally, as I understand, this flavor of UNTIL actually applies to the IO thread, while the traditional one only affects the SQL thread. It reads between the lines in the abovementioned KB article, but it would be nice to emphasize it i it's true.