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.