I'm not the original person, but I've had exactly this happen before on both Arch and NixOS. Long ago when I was on Ubuntu I believe this also happened when I tried installing KDE (rather than wiping and installing Kubuntu). I've recently seen recommendations from people saying that if you're going to try to have both GNOME and KDE installed alongside each other, to keep one user only on one, and the other user only on the other so that their config settings don't get intertwined.

However right now I'm on Fedora Silverblue, I was on Kinoite and did a rebase to Silverblue (which means I went from KDE -> GNOME) and the only issue I had was a few icons were broken, which was resolved through setting it back to Adwaita in Tweak Tool. I'm guessing the fact that the rebase caused all of the KDE packages to get removed while installing the GNOME packages made it conflict less "violently" so to speak - which also had the nice effect of not having a bunch of duplicated apps as well.