Starting with plasma 5.27.x I have problems with HiDPI settings.
I use a DELL XP13 notebook with an internal 4 K display (3840x2400 16:10) and
an externel display with 2560x1440 (16:9).
To achieve the best results for usage with and without the external display I
wrote a small script, that uses xrandr to find out, if the external display is
connected or not. If it is not connected, so that the internal display is the
only one, the script is setting:

GDK_SCALE=2
GDK_DPI_SCALE=0.5
XCURSOR_SIZE=48
SPOTIFY_SCALE=--force-device-scale-factor=2
QT_AUTO_SCREEN_SCALE_FACTOR=0
QT_SCREEN_SCALE_FACTORS=eDP-1=2
PLASMA_USE_QT_SCALING=1

Except for the XCURSOR_SIZE this worekd perfectly up to plasma 5.26.x, but
stopped working in 5.27.x.
The script works as usual, but it seems as if the variables are ignored now by
plasma and all KDE applications.
For GTK I read, that GDK_* variables are no longer used with plasma 5.27.x, but
I couldn't find anything regarding the QT_* variables.

As long as this isn't solved, my notebook is completely unusable without the
excternal display.