When you specify UXA manually through the xorg.conf snippet that overrides the GPU autoconfiguration, and disables activation of the nvidia GPU and sharing with nouveau through PRIME. When you leave it at its defaults, then it explodes inside nouveau.
Also it appears that you are experiencing the plymouthd race.
Still not an SNA bug :-p
When you specify UXA manually through the xorg.conf snippet that overrides the GPU autoconfiguration, and disables activation of the nvidia GPU and sharing with nouveau through PRIME. When you leave it at its defaults, then it explodes inside nouveau.
Also it appears that you are experiencing the plymouthd race.