In Falkon About it still says QtWebEngine 5.15.2, probably it should be rebuild against newer QtWebEngine versions? It has issue with 5.15.4
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No, 5.15.2 is correct, qmake.conf is patched to set to 5.15.2 to keep it compatible with the rest of Qt 5 and those that depend on exact versions.
What pages have issues? Kmail and Akregator correct?
Reddit (when scroll and content should be updated) other dynamic pages.
It can't open youtube.
None of your issues are reproducible on a normal, fully updated KaOS system (ofc with the [build] repo, otherwise no qtwebengine 5.15.4).