Hello Richard,
I just had the same problem.. I dont know where it came from but my hypothethis is that the first password i picked during first install was not filling requirement somehow and made mariadb bug. Unfortunatly no way to change it cleanly , I had to remove the container , remove the directory and files for my maria db in my appdata share then clean reinstall using fresh from the start "password" as password.
[RESOLVED] Afterward i still get error when i try the modification of the password through mysql console that i dont have permission of updating my password. Really weird.
Ok i found how just a synthax error ( i was using 'root'@localhost') : command to change root password:
MariaDB [(none)]> ALTER USER 'root' IDENTIFIED BY 'new_password';