Social Login: username with special characters not taken from keycloak

Hi community,
I am facing problems with keycloak users with special characters (german “Umlaut”) in their username. While the authentication works and the user can log in, the username is not taken over from keycloak at the first login. Nextcloud sets some cryptic letter-number-combination as username instead. I met this behavior earlier for all users before I created a user property protocol mapper in keycloak which adds the username to the access token. Now for users without special characters in their username, the username ist set to “keycloak-”. But, as said, not for usernames with special characters.
Is there any workaround or does nextcloud simply not support usernames with special characters?