Created attachment 156776 [details]
kscreen-doctor -o

SUMMARY
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STEPS TO REPRODUCE
1. Connect two monitors, one 4k, one 1920x1200 (or similar)
2. Login to a desktop session
3. Inspect apparent font sizes in KDE and GTK application windows

OBSERVED RESULT

The KDE and GTK application fonts are tiny in comparison to 5.26.*

EXPECTED RESULT

The KDE and GTK application fonts are the same size as in 5.26.*

SOFTWARE/OS VERSIONS
KDE Plasma Version: 5.27.1
KDE Frameworks Version: 5.15.8
Kernel Version: 6.1.12-1-default (64 bit)
Graphics Platform: X11
OS: OpenSUSE Tumbleweed 20230225

ADDITIONAL INFORMATION

I went to KDE settings->Workspace->Startup and Shutdown->Background Services and disabled the KDE service, and now everything is back to normal.

This happens on both a standard configured login, and my personal login that differs in the following respects:

1) I disable kscreen2 to prevent hotplug actions from moving applications from one display to another.
2) I configure the multiple displays both using Settings Display and Monitor and also nvidia-config.
3) I have /etc/X11/xinit/xinitrc.d/50-xrandr.sh run 
    xrandr --output DP-4 --auto --output DP-0 --auto --scale 1.75x1.75 --right-of DP-4

Regardless of whether I use a standard login or my personal customised login, to achieve the same font sizes as 5.26.*, in 5.27.1 I have to disable the GNOME/GTK Settings Synchronisations Service.

I suspect the GNOME/GTK Settings Synchronisations Service, I suspect it is overwriting .config/xsettingsd/xsettingsd.conf.  At one stage I tried manually setting the DPI in xsettingsd.conf.  But when I login, the config file gets changed back to 98304, which means 96 dpi (98304/1024=96).