Currently multimonitor support in SDDM is broken. Upstream report: https://github.com/sddm/sddm/issues/496 Reproducible: Always
Created attachment 415928 [details, diff] upstream patch
There is now 0.13.0 in portage, can you confirm this is fixed?
0.13.0 seems to be ok. On the problem version, I also identified that deleting the configuration file /etc/sddm.conf allowed it to work again, but unfortunately I didn't identify which setting caused the problem (I had changed the theme via system settings, nothing more). I can confirm that changing the theme in 0.13.0 is working OK here, so far!
The upstream patches are part of 0.13.0 which we ship.