I think this is something to do with MariaDB strict mode, which is on default from 10.2.4.
"With strict mode set (default from MariaDB 10.2.4), the statements will fail, and an error will be returned instead. The IGNORE keyword can be used when strict mode is set to convert the error to a warning." https://mariadb.com/kb/en/library/sql-mode/#strict-mode
This seems to be problem only in registerations, so maybe this is a bug there.