Yes, it sounds likely to be here the origin. Wayland’s Mutter (GNOME) is a totally different beast than Wayland’s Kwin (KDE/Plasma). So I’m not surprised to read Takiro having a better user experience over there than you, Gerhard.
Wayland delegates so many things to the compositor compared to X11, afaik. So, I wouldn’t be surprised if the support and quality of is totally night and day from one DE to another at this point.
Bravo for testing this part, that was the right thing to do. I’m sorry if you don’t have a solution other than moving away from GNOME Mutter, or wait for a fix (it can take long). That’s a serious papercut. As a workaround, you can probably change your habit of drag’n’drop layer with one based on keyboard shortcuts. The layer stack has many assigned keyboard shortcut, and you can move the layers this way. Not ideal, but it can maybe help if your needs are not that big in terms of moving them around.
Yes, I tested last week, and I don’t exclude adopting Wayland Plasma somewhere in 2025 now. The quality greatly increased over the last 5 month. It even has now a tablet option that was one of my feature request for a decade (limit/cap low or high pressure level) and X11 don’t have it. 
For the one who prefer to load their option via a bash script like with xsetwacom in X11, a utility ktabletconfig exist now. GNOME made their own equivalent gsetwacom. (edit: and cherry on the cake, this command line are now persistent, and don’t need to be reloaded each time at startup or after sleep.)