KDE desktop prefers its own display settings over xrandr?

Question

I have a set up with three monitors for KDE Plasma (5.12). One is a HiDPI monitor which I have scaled at 2x, and I have forced the other two ("standard" resolution) monitors to use their native resolution unscaled via the following xrandr script:

xrandr \
    --output DP-1 --auto \
    --output eDP-1 --auto --scale 2x2 --below DP-1 \
    --output DP-2 --auto --right-of DP-1 --scale 2x2

This gives exactly the configuration I want. However, for whatever reason, while I can place windows anywhere within these screens, the KDE desktop is confined to just the unscaled pieces. In other words, my desktop currently looks like this:

I wouldn't care much if this was just the background image being the wrong size, but things like the right click menu only work inside of the "desktop" corner as well. As far as I can tell, this is because KDE clings to its own Display settings, which says the screens are still un-scaled, rather than xrandr.

Any ideas on how to fix this?

(P.S. I'm aware that the Plasma Wayland session has per-display scaling, but I use a lot of Xwayland applications that appear fuzzy on my HiDPI display when using the Wayland session).