Hello wroot,
i have investigated this issue. The commands above are working fine and are definitely the way to go. In my case the issue was that i had a Option-Group for my MySQL server wich was overwritten every time a reboot or maintainence happens. Defined a permanent Option group for my db (character_set_filesystem, character_set_client, character_set_connection,character_set_database, character_set_results, character_set_server -> all to utf8mb4) and (collation_connection, collation_server -> all to utf8mb4_unicode_ci) everything works. The tables were utf8mb4 as described above but without the option group settings it couldn't work.
Sorry for confusing