some additional observations:
If I create a Network with network type "vlan" the DHCP assignment works, but only if the VM is on a vESXi with a controller vm.
With network type "flat" it doesn't work either.
So this seems to be more a network related issue, maybe because its nested.
Ok I could fix this:
Issue "vlan based network on different nested Hosts":
I've to enable VLAN Trunking on the underlying vswitch.
Issue "flat network not working":
For DVS the documentation states: "With VDS networking, you can create a VLAN-based provider network." because for NSX deployments the term "flat" is explicit included in this statement, I conclude that "flat" is not supported for VDS.