I have checked in with @sharpd, he mentioned that it might not be supported to use v4 nexthop for v6 route.
From local testing:
vova | ip r default via 192.168.0.1 dev wlp4s0 proto dhcp metric 600 192.168.0.0/24 dev wlp4s0 proto kernel scope link src 192.168.0.102 metric 600