Mariadb concurrent insert not working for MYISAM tables
Face it, MyISAM survives only because it uses table locks for almost every kind of write.  Even a SELECT can get caught up in the middle.
The obscure exception has to do with multiple INSERT that are inserting into a table with AUTO_INCREMENT PRIMARY KEY (and, I suspect, no other UNIQUE keys).  In that case, they can play around at the &quot;end&quot; of the table with less locking.
Switch to InnoDB.
If your goal is to ingest lots of data from lots of sources, here's a technique:  http://mysql.rjweb.org/doc.php/staging_table