TLDR: No DHCP offers from pfsense with VLAN tagged on a VLAN-aware host. Switch is trunked to allow "All" traffic.
I have a VLAN tagged as 20: https://i.imgur.com/Rz61DMz.png
And its corresponding interface: https://i.imgur.com/pQxJitG.png
And its DHCP server: https://i.imgur.com/jsAjxid.png
I start packet capture to see if DHCP on this VLAN responds: https://i.imgur.com/xlfsm4U.png at Diagnostics -> Packet Capture -> VLAN20 (vmx1.20) (I don't see any activity, zilch).
On proxmox side, I have this guest: https://i.imgur.com/md46zBT.png
Its host has bridge-vlan-aware yes: https://i.imgur.com/72iiWmW.png also the GUI: https://i.imgur.com/L2WBcgo.png
I start data capture on the host to see if DHCP for VLAN 20 is being solicited (it is): https://i.imgur.com/HMdpzqi.png
The guest is timing out, with no DHCP offers: https://i.imgur.com/Yyul1iA.png
Then finally, on the switch's port, I double-checked that it's allowing All: https://i.imgur.com/iTnIzH2.png