In Fedora, Plasma 6.1 still treats my 5k Thunderbolt screen as 2 distinct screens (hardware implementation detail to overcome 4k limit) when arranging displays or showing overviews. Gnome treats it as one screen.
I think the best is to submit the bug report to KDE bugzilla so the devs could look at it.
Installed on my openSUSE Tumbleweed and Gentoo computers and so far Plasma 6.2 working great
Just installed on Gentoo and it is working great so far