No longer able to input circumflex or diaeresis accented characters (qt5 apps only !)

Currently running KDE-Plasma-5.23.5 (X backend)/ KDE-Frameworks-5.90.0 / QT-5.15.3
I can assert that this used to work perfectly but am unfortunately unable to assert from which version of what the problem started to occur systematically. ---------- Whatever the qt5 application I launch (kmail, kate, konsole, dolphin, qtdesigner…) I am no longer able to key in the â ä ë ê î ï ô ö ü û (as well as corresponding uppercase) characters. (usually possible via hitting the dedicated dead key then the desired vowel) Keying any other directly accessible accentuated character works fine. I suspect this problem to be restrained to qt5-ed apps since it does not impact non qt5-ed applications such as The-Gimp, Firefox, Chromium, Yoshimi (fltk), my Arduino prototyping platform, audacity… all launched from the same user under the same DE. ---------- **UPDATE 1 / XMODIFIERS :** Curiously enough, If I export XMODIFIERS= before launching my DE, or even within some konsole session, then, if I launch kate *manually* (via bash), the problem does not occur.
It however still occurs when starting kate from the KDE menu or clicking on its icon.
Therefore my problem would become, how should I set XMODIFIERS so that whatever qt5 app could automagically honour it ?
BTW, unless I export it explicitly, this variable is not part of the environment when my DE is running.