Virt-Manager - NIC link state set to down, but guest network still connected

If the problem persists on a Linux system, it might be related to the VM configuration.
Please post the relevant parts of your configs:

virsh list --all
virsh dumpxml vm_name | xmllint --xpath /domain/devices/interface -

sudo virsh net-list --all
sudo virsh net-dumpxml vnet_name