My implementation can be found here:
https://gitlab.com/gwj76/game/-/blob/feature/networking/Scripts/RelayMultiplayerPeer.gd?ref_type=heads
#MultiplayerPeerExtension
viral mirage
please notice that the word "relay" in is_server_relay_supported is different from the the "relay" in RelayMultiplayerPeer.
viral mirage
Ah ok I think i've figured out why this happened.
The server was registering itself as a connected peer. And for this reason it was adding itself to the relayed peers