Reproduction :
1. Set keyboard layout Ergo-L (for instance), assuming you have a recent xkb
2. focus a text input in a Wayland window
3. press key [O] (supposed to be the level5 latch in Ergo-L)
4. press a second time (level 5 of [O] is dead diaeresis in Ergo-L)
5. press key [D] (character "e" in Ergo-L)

Expected:
4. should display an indicator that dead diaeresis is active
5. should output character "ë"

Observed:
4. nothing
5. character "e" (without diacritics) is output

This has been happening at least since plasma 6.1.4.

It does not affect:
- X11 apps
- level 5 symbols on other keys
- symbols on other levels than 5 on the key havingy ISO_Level5_Latch on level 1

More data : I checked the changelog of version 6.1.4  and noticed some work concerning level latches. Could it be related ?
Suspected change: https://invent.kde.org/plasma/kwin/-/commit/0374acb1d82e827c254eeb1444cb0ce15aee635c#1a71db6ed299b894bf188ebe6a7a4cc093278abf_1030_1030