Fixed! Minor difference in how Damus sends a Host header (for default ports, damus sends hostname:port, even for default ports) compared to other clients. My HAProxy rules were doing an exact match for the hostname, changed it to a prefix match, and all works!

If you used my sample docs to setup HAproxy for a relay, check out this patch that ensures Damus requests make it to the correct backend: https://git.sr.ht/~gheartsfield/nostr-rs-relay/commit/1595ec783d1312206c0a1d7c66b5bfa1ff83f3a4