Are you certain it hasn't started on some TTY other than 8? I always start out on TTY1, although I start up text only, no SDDM. However, I do have a very vague memory of something similar, and I believe it was that I needed to change one of the kernel FB related settings. Sorry not be have more concrete suggestions.
I'm having a problem I'm not quite sure how to tackle: every other startup or so results in a black screen. Usually nothing gets printed (no bios splash, not GRUB menu, no OpenRC prints), and the monitor goes to low power mode after a while (I haven't quite confirmed if this is actually the case, or if everything happens before my monitor have actually stated, but I'd expect the GRUB menu would hang long enough for it to do so).
However, it does wake up if I switch to another TTY (e.g. ctrl+alt+F4) and lets me log on, so it has obviously booted up. If I switch back to TTY 8 from there it just shows a blinking cursor (i.e. not SDDM, which is what I'd expect). If I reboot from the TTY that lets me log on, the boot process is usually normal and leaves me at the SDDM login.
Any tips on how to debug this would be much appreciated.