According to the changelog of gnome-keyring gnome-keyring should autostart with
sddm, but this is not the case. The relevant line I have in common-session-pc
is:

session    optional    pam_gnome_keyring.so    auto_start
only_if=gdm,gdm-password,lxdm,lightdm

If I add "sddm" to this, it is working. The spec does have a line that includes
"sddm", but somehow it is not written that way.

The spec includes the following comment:

###
# WARNING: when changing the pam-config calls in the scriptlets, please also
# update them in baselibs.conf.
# pam-config checks if the pam module is available for 32bit too if pam-32bit
# is installed, so the call here might fail until gnome-keyring-pam-32bit gets
# installed (see bnc#728586).
###

I have the 32bit package installed and the baselibs.conf is not updated to
include "sddm", which is probably the problem.