Hi,
I know it's a consumer hardware/computer, not a server.
When you tried this, the "other" has the same L2 connection as the proxmox server ?
Well, I don't know your background, but I know that the same setup is working in my case, so I know that it's not simply a bug in proxmox (and you will not be the only with this issue).
But that is fine if you don't want to spend any more time and this, i just wanted to try to help since this issue is weird and for me not related to proxmox (based on the information provided).
Best regards,