Dead keys used in non-english keyboard dispositions to enter diacritics (e.g. êîûëïü) fail to work for native Qt applications under Wayland (X11 works fine). It also fails when using Qt's Virtual Keyboard.
In my case, its a french keyboard disposition, but this has been reported with different dispositions in the following recent KDE bug reports (although it is most likely a Qt issue):
KDE bug 411729
KDE bug 426621
Steps to reproduce: open a native Qt5 application under Wayland (e.g. kate under plasma wayland) with (say) a french keyboard. Press the key ^ followed by the letter e.
What happens: an e is entered.
What should happen: it should be an ê.
EDIT: removed the side note about qtVirtualkeyboard