Well, they suggested to run kcmshell6 qtquicksettings and put it back to automatic since for some reason your system runs on software render. Also upstream claims to have fixed the issue in Qt 6.3.1 for qtwebengine. KDE might need to adopt the same fix for plasma on software render. So I will suggest to bump the bug report once more to see what KDE developers might do to solve it.
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