Ah.
The thing with online raids is that you can host the same one multiple times, which can be used to host shiny raids. Also, finding a shiny in an online raid means that the host found the shiny in their game. So either you found a raid someone was hosting multiple times, or you found a raid which just happened to have a shiny. However, since you joined random raids, I don't know what the chances of finding a shiny in an online raid would be. I imagine it'd be quite low.