Sporadically Host is not allowed to connect to this MariaDB server when Create User if not exists

Sporadically MariaDB returns Host '<IP>' is not allowed to connect to this MariaDB server
This happens if the user is already existing but a CREATE USER IF NOT EXISTS / FLUSH PRIVILEGES; is called on the logon user.

About 95% of the connections do work, 5% results in the error above.

mysql.user:

• mariadb.sys@localhost
• root@localhost
• app@%

Application is connecting using user app

Log in MariaDB:
2022-03-10 9:05:36 8 [Warning] Aborted connection 8 to db: 'unconnected' user: 'unauthenticated' host: '192.168.48.3' (This connection closed normally without authentication)

MySQL 8 does not show this behavior and but is logging a warning on the console.

Idea
It seems that MariaDB is somehow recreating the user even if it exists. That may guide to the fact that - meanwhile - another connection is not possible for this user.