No, systemd-homed was not the issue. I needed to add the option ignore_unkown_user to /etc/pam.d/common-account.
Unfortunately this option is not supported by pam-config yet, so I had to apply a manual change to the PAM configuration.
/etc/pam.d/common-account after the change:
account sufficient pam_systemd_home.so
account requisite pam_unix.so
account sufficient pam_localuser.so
account required pam_sss.so use_first_pass ignore_unknown_user