Funny, I did SSE with Servant just the other day: my post.
At one point, I experienced the same thing as you did - the client didn’t seem to receive anything. However, when I forcefully shut down the server all sent messages were displayed! It turns out that Firefox wouldn’t show the continuous stream of messages until the connection was closed.
You can check out the final handler sseH here: link to handler, source. I know nothing about Snap, however.