Tested on COMSIC Desktop Epoch 1 Beta 1, under which Qt currently uses Client Side Decorations.

Reproducer is very simple:

#include <QApplication>
#include <QMessageBox>
#include <QTimer>

int main(int argc, char** argv)
{
    QApplication app(argc, argv);
    QTimer::singleShot(0, &app, []() {
        QMessageBox::warning(
            nullptr,
            "A dialog box",
            "The file Untitled has been modified.\nDo you want to save your changes?",
            QMessageBox::Save | QMessageBox::Discard | QMessageBox::Cancel);
    });
    
    return app.exec();
} 

When the dialog first appears, some of the text is clipped and buttons are shrunk and overflowing - see "bad" screenshots. After interacting with the window frame in some way (i.e by trying to resize or maximizing and then restoring) - it is then shown properly.

This is identical in both the bradient and adwaita decoration plugins (looks slightly worse on adwaita).

Does not happen under GNOME as shipped with Ubuntu 22.04 LTS - in there bradient decorations look OK from the start, but Adwaita decorations are broken (though in a slightly different way). This matches https://bugreports.qt.io/browse/QTBUG-137253 - so this one is probably a different issue.