After a couple of hours debugging why my Proxmox LXCs had no outbound Internet, I found that they were sending DHCPDISCOVER packets but never receiving a DHCPOFFER.
Software resets didn’t help. Neither did rolling the UniFi switch back to older firmware.
What finally fixed it was physically disconnecting and reconnecting the 10GbE SFP+ link, even though UniFi showed it as up the entire time.
I believe this is the second time I’ve seen this behavior.