The default group membership for the first user on a Manjaro Linux installation is
# addgroups="lp,network,power,wheel"
– manjaro/kde/profile.conf · master · Profiles & Settings / iso-profiles · GitLab
The profile for building the Gnome edition is extending the default
addgroups="lp,network,power,wheel,users,storage,input,audio"
– manjaro/gnome/profile.conf · master · Profiles & Settings / iso-profiles · GitLab
This difference is a maintainer decision - it is how it is, no questions asked, no blame casted.
The copy/paste from a KVM guest to the host depends on the membership of the input group.
Thus for the issue at hand - a Manjaro Gnome will behave different than a Manjaro Plasma virtual machine.
Inside the Plasma guest - add the user to the input group, and the copy/paste from guest to host will be activated upon next sign-in.
sudo gpasswd -a $USER input
Remember to sign off and sign in to activate the group (or reboot).