I have rawhide installed in a kvm vm.  Both plasma and gnome desktops are installed and sddm is
the DM.

After logging in to a gnome session the screen will resize to fill whatever the VM's window is.
No matter if the VM's window is a standard resolution or not.

When logging into a plasma-wayland session the screen will not resized and one has to use the
display settings to pick a "close enough" resolution.  This is so, even though the sddm login screen
fills the non-standard display size.

Additionally, a non-wayland plasma session does what gnome does and what I believe is the
preferred behavior.

Would I be correct in thinking that this would be a bugzilla against kwin-wayland?

I'd try that, yes. You might have better luck reporting it upstream rather than in Fedora.