Consul watchers (websockets or sse)

I am testing consul watchers, the consul cluster is behind an HAProxy but I am frequently getting this error:

# Watch errored: type=key error="Unexpected response code: 504" retry=5s

For now, I increased the timeout server in the backend to 1h, but wondering why not something like WebSockets or SSE was implemented for this (to keep the connection alive something like a ping/pong)?