While I agree that full DHCP conformance would be nice, kernel ip autoconfiguration, especially in combination with netboot/nfsroot is a rather special case. The assigned IP addresses have to be static and permanent.
Consider what would happen if the DHCP server wouldn't renew your lease or give you a different IP. Basically all the client could do in that case is panic - you can't just shutdown network (as the rootfs would be lost) or unmount the NFS root and set up a new IP (when your NFS root is gone so are ifconfig, mount etc).
It's best to see the kernel DHCP implementation as a nice shortcut so you don't have to manually specify the network parameters in the ip=... kernel option, more like BOOTP or RARP.
See also the discussion in the predecessor of the linked PR connman: ignore kernel-configured netdev and improve resolv.conf handling by HiassofT · Pull Request #3883 · LibreELEC/LibreELEC.tv · GitHub
so long,
Hias