Has anyone resolved this problem? I am having the same problem. I have a local Debian DHCP server on the same subnet as the proxmox host. The KVM DHCP request reaches my DHCP server which attempts a response, which never gets to the KVM. I have tried turning off all the firewalls and have deleted the NIC on the KVM and re-added it. A new Debian install in KVM still can't get the IP address. See screenshots below documenting my setup and what is happening.
Finally figured it out. I was running proxmox second level under ESXI host 6.0. I installed same version of proxmox on bare metal and it works fine with the exact same settings.