Compose key does not work on Xorg anymore in Qt 6

I'm reporting this as a user without any development experience with Qt. I was redirected here from https://bugs.kde.org/show_bug.cgi?id=487302.

I am running Plasma Desktop on Arch Linux. Since upgrading to Plasma 6, my compose key has stopped working. This applies to all KDE applications, for example dolphin, konsole, krunner and kwrite, but also to other applications such as lowriter, stream and qgit. It does still work with chromium, firefox, gnucash, signal-desktop, element and vscode (I think those are all using GTK).

I am not sure whether the non-KDE applications already had the problem before the upgrade, but the KDE applications definitely did not.

I have Caps Lock configured as the compose key. It seems like in the affected applications, pressing Caps Lock is simply ignored. It neither enables actual Caps Lock nor does it initiate a compose sequence, the following characters are simply typed as pressed.

This is the output of setxkbmap -print -query:
xkb_keymap {
xkb_keycodes{{{}

{}}};
xkb_types { include "complete" };
xkb_compat { include "complete" };
xkb_symbols { include "pc+us+de:2+cz(qwerty):3+es:4+inet(evdev)+compose(caps)" };
xkb_geometry { include "pc(pc105)" };
};
rules: evdev
model: pc105
layout: us,de,cz,es
variant: ,,qwerty,
options: compose:caps

I am running these versions:

Linux: Arch Linux
KDE Plasma Version: 6.0.4
KDE Frameworks Version: 6.1.0
Qt Version: 6.7.0