When I boot and start a Plasma session, the environment variable
QT_SCREEN_SCALE_FACTORS is set:

   ~> echo $QT_SCREEN_SCALE_FACTORS
   eDP-1=1.5;DP-1=1.5;HDMI-1=1.5;DP-2=1.5;DP-1-1=1.5;DP-1-2=1.5;DP-1-3=1.5;

AFAIU this is correct: I did select 1.5 screen scaling via the GUI earlier.

When I boot and start a Plasma Wayland session, the same variable
is not set.  Again, the seems to be correct: Wayland apparently
communicates the screen scaling by other means.

However, when I boot, and then
1) start a Plasma session
2) log out without reboot
3) start a Plasma Wayland session

then QT_SCREEN_SCALE_FACTORS *is* set.  And this leads to all sorts
of dubious effects, because now the X11 and Wayland scaling mechanisms
interfere.

Apologies if I am reporting this for the wrong package.  Please correct me.

Likewise if this is really an upstream problem that should be reported there.