You are doing nothing wrong, it's just unusable, if you can report it over there https://gitlab.gnome.org/GNOME/gimp/-/issues > maybe, they will try to fix it.
30-second video > https://imgur.com/q0lnQ8e
The "only" thing I came across is to shrink the selection before Ctrl+C, but even though, it's far from perfect as you can see below
I also noticed changing the original/default size will worsen the black dirt
30-second video > https://imgur.com/q0lnQ8e
The "only" thing I came across is to shrink the selection before Ctrl+C, but even though, it's far from perfect as you can see below
I also noticed changing the original/default size will worsen the black dirt