I'm not sure where to start investigating this.

I'm using Plasma Desktop on Arch Linux with Xorg. I have Caps Lock configured as my compose key.

Since upgrading to Plasma Desktop 6, the compose key does not work in KDE applications anymore, for example in dolphin, konsole, krunner, kwrite. The compose key still works in chromium, firefox, gnucash, signal-desktop, element, vscode. Interestingly, it also seems to have stopped working in lowriter, steam and qgit.

In the apps where it doesn't work anymore, pressing the compose key will simply have no effect. It will not enable caps lock and the following letters will just be typed as normal.

This is the output of setxkbmap -print -query:
xkb_keymap {
        xkb_keycodes  { include "evdev+aliases(qwerty)" };
        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

SOFTWARE/OS VERSIONS
Linux: Arch Linux
KDE Plasma Version: 6.0.4
KDE Frameworks Version: 6.1.0
Qt Version: 6.7.0