I did some packet captures on the DHCP server to make sure the router gives away a default gateway, and everything seems correct.
I've dug around systemd-networkd for a while, enabling debugging (https://superuser.com/questions/1187633 ... d-networkd)
and checking out similar reports (https://github.com/systemd/systemd/issues/1982), but it didn't help.
I tried out netplan with networkd backend and I still didn't get a default gateway and then I installed NetworkManager (
sudo apt-get install network-manager), rebooted and now I do get a default ipv4 gateway...
So - I leave this as a warning for others hitting the same problem... Must be something triggered by my environment (my DHCP server also injects some static routes).