The ssh_keys snippet comes after the post_s390_reboot snippet in the kickstart, so .ssh/authorized_keys never get written to disk.
This affects ks_appends, the OS-family-specific posts, and the system_post as well as ssh keys.
This bug has been addressed in Beaker 0.9.0-3 which is currently running on stage: https://beaker-stage.engineering.redhat.com/
Beaker 0.9.0 has been released.