The issue here is the same one that happened in gdm last year where the startup would race with the load of DRM graphics drivers.
GDM fixed it by essentially running the equivalent of 'udevadm settle'. The same kind of fix needs to be ported to sddm or systemd needs to be modified to not emit CanGraphical until DRM graphics drivers are fully loaded.
The issue here is the same one that happened in gdm last year where the startup would race with the load of DRM graphics drivers.
GDM fixed it by essentially running the equivalent of 'udevadm settle'. The same kind of fix needs to be ported to sddm or systemd needs to be modified to not emit CanGraphical until DRM graphics drivers are fully loaded.