User added to wheel group, but can't su without password authentication

On a SLES 15 SP1 system, I've added a user to the wheel group:

wheel:x:1003:joeuser

however, when joeuser logs in as themselves, then tries to su (to become root), they are asked for their password. Why could that be happening? Here is /etc/pam.d/su:

#%PAM-1.0
auth     sufficient     pam_wheel.so trust
auth     sufficient     pam_rootok.so
auth     include        common-auth
account  sufficient     pam_rootok.so
account  include        common-account
password include        common-password
session  include        common-session
session  optional       pam_xauth.so