I don't know. Perhaps it is crashing for some reason? I did see that one fix I had high hopes for, was committed to dev and the 6.9 tree but for some reason was NOT included in 6.9.1 (according to the qtwebengine github site). It has a good chance of helping across all gpus since the underlying issue was missing share graphics buffer permissions.
See
https://bugreports.qt.io/browse/QTBUG-136257
I just noticed that it was omitted from 6.9.1 and asked why? No idea if they will respond. I am hoping Arch maintainers will cherry-pick that fix since it was committed to both dev and the 6.9 branch.
And then someone would have to disable the patch that turns off GBM for nvidia for anything to help.
Sometimes I think one end of Qt has no idea what the other end of Qt is doing. And the Qt people who triage new issues seem to be clueless as well.