Logging into the KDE Plasma (Full Wayland) session after having logged in and out of the Plasma (X11) session causes scaling in the Wayland session to be broken. Rebooting and logging into Plasma (Full Wayland) without logging into the X11 session first works fine.

In particular, my setup normally has the screen scaling set to 200%. This works fine in Wayland. In X11, applications also detect the DPI and scale themselves, and this largely works.

When logging out of X11 and into "Full Wayland", the entire session acts as though the scaling is set to 100% (though it still shows up as 200% in systemsettings5, everything is tiny). Changing the scale to something else and back "fixes" the issue, but leaves several artefacts: a number of applications and desktop widgets (possibly just Qt-based things: KRunner and Calibre are two examples) scale themselves again, resulting in them running at effectively 400% size.

Logging in and out does not fix the issue, but rebooting does.

There don't appear to be any differences in the Qt environment variables set between working and not-working wayland setups. X11 is quite different. The relevant-looking ones are:
QT_AUTO_SCREEN_SCALE_FACTOR=0 (on both X11 and wayland)
PLASMA_USE_QT_SCALING=1 (wayland only)
QT_WAYLAND_FORCE_DPI=96 (wayland only)
QT_SCREEN_SCALE_FACTORS=eDP-1=2;DP-1=2;DP-2=2; (X11 only)

I haven't had a chance to try this on another distro, but am happy to report it upstream to KDE if it turns out to not be specific to SUSE's configuration.