Same problem here with my HDMI screen. Everything works fine (GNOME Wayland
session and all) until I start an X server (not XWayland, actual X server from
a "GNOME on Xorg" session) at which point the screen goes black (X server is
mode-setting I'd imagine), and when it comes back on the purple line is
present.
Logging out of the Xorg session and back into a Wayland session does *not*
revert the problem; I haven't tried the xrandx trick yet.

The screen is indeed blurry (enough to make it unbearable to read any text with
subpixel positioning for more that a few minutes). It might comes from the
scaling of the output to the remaining space (screen space - purple line
space). Furthermore, any screenshot does not capture the line and it's only an
'on screen' thing.