The keycloak package prior to version 17.0.1-2 was running with WildFly server. Since upstream officially moved to Quarkus distribution, Arch Linux follows this approach. This means some manual intervention is required for the upgrade.
The configuration needs to be adjusted from the old .xml format to the new /etc/keycloak/keycloak.conf.
Prior to the upgrade, stop the keycloak service, upgrade the package and migrate the configuration before starting:
systemctl stop keycloak.service
pacman -Syu keycloak
# migrate configuration /etc/keycloak/keycloak.conf
systemctl start keycloak.service
See Keycloak migration docs and Keycloak Quarkus server docs