by any chance, is this a hosted setup?
as in the Proxmox host is in itself a VM?
I had similar problem when I run my test setup "a proxmox host in a hyper-v VM"
in my case, I needed to set my virtual nics to mac spoofing and make sure that my hardware nic was set to promiscuous mode as well.
if your setup is a real hardware, make sure the nic is set to promiscuous. not sure if you can have mac spoofing on proxmox.
it is still looks like bad config.
please drop the host and VM network config here.