I confirm the problem, but seems to be on Google’s side. Sometimes site:discourse.julialang.org brings less than 10 results, while other times it brings dozens. Of course recent topics don’t appear yet.
I am going to look into setting up SEO for this subdomain. I can confirm however that we are visited by Google’s crawlers and I would expect that once we see more incoming static link the prevalence of results from this site rises.