I don’t think this is an Incus issue so much as all of the listed instances where you’re not getting an address are those which use NetworkManager.
NetworkManager has had some issues getting addresses on macvlan devices before so that may be what’s going on here.
Not sure why busybox isn’t happy but that one may be because it just doesn’t do network config out of the box.
Alma, CentOS, Rocky, … all will have the same issue as they all use NetworkManager.
Most of our working images use systemd-networkd so that may be a good alternative here even if the only fully supported option on those distros would normally be NetworkManager.
Excuse my unawareness but neither netplan nor systemd-networkd are available in EL9, as far as dnf shows me in a search. Also i have been unable to find any articles online about how netplan might aid in converting NetworkManager rules to systemd-networkd