These are my specs:
Fedora release 30 (Thirty)
plasma-desktop 5.15.5-1
First display: laptop screen 2880x1620 (retina, hi-dpi display, scaled 2x)
Second display: LG 23EA52 1920x1080 (via HDMI-2)
Since the LG display is not hi-dpi and plasma-wayland still has some quirks (so, I'm not using it), I have to rescale the display via xrandr:
xrandr --output eDP-1 --auto --output HDMI-2 --auto --panning 3840x2160+2880+0 --scale 2x2 --right-of eDP-1
but the problem is not due to the rescaling.

The problem:
Since the last (or second to last) update, I lost all mouse click actions (left, right & middle) on dolphin and some other KDE apps (such as okular) when the app is on the second display. I can still browse directories and activate actions with the keyboard, though. If I move the app to the first display, mouse actions are restored.
Non-KDE apps work, such as web browsers (firefox, Vivaldi) and, funnily, some KDE apps too, such as Apper and System Settings.
Any idea on what could be causing this?