Also – tried using the suggestion above but replacing kwin_x11 from root leads to incorrect settings loading, defaulting to higher dpi so I suppose that’s not really right solution here. Downgraded to 6.7.6_14 amd64 for the time being.
In addition to what I wrote above, it obviously does not work fine with 6.8.1 — it did work fine with the earlier 6 series releases.
I have the exact same issue on Kubuntu and KDE Neon. Also here – it worked fine with 6.8.1. Just wanted to let you know that something did indeed change and to me it does act up like a bug (unexpected behavior).