I had that issue when trying to connect the nodes of a k8s cluster to nm network as external clients. As soon as I turned wireguard on with wg-quick it would break internet connection. I was using calico which was also making changes to network and iptables. I could not figure out the issue but after changing from calico to cilium my issue was gone. So maybe it could be related with your network config or some other tool that is making changes to it.