Here is an excellent workaround for those using KDE Plasma. It also works on GTK apps. Why a workaround is required and each user has to fix each application themselves is anyone’s guess - but the workaround is fairly simple. I found it here:
Fix Generic Wayland Icon
- Open the app you are wanting to fix (Vivaldi, Firefox Nightly, HandBrake, etc.)
- Add new kwin rule with Alt+F3 → More Actions → Configure Special Application Settings → Add Property → Desktop file name →
- Enter the desktop file name without extension (e.g. chromium-browser.desktop, fr.handbrake.ghb.desktop, vivaldi-stable.desktop, etc.)
- If you are using standard package, check name in “/usr/share/applications/”
- if you are using flatpak package, check name in “var/lib/flatpak/exports/share/applications/”