The release of the UFO’s happened during my (only) 6 month break to focus on schooling and wait for new content (almost everything was bright released and had some sort of fancy event in that period… ), but after that, I can say this, just as another outlier data point:
I have never been in a Realm with the UFO’s, personally. However, my sister, who uses my account as well, has seen quite a few over time. Thinking about how many realms that I’ve been in since return vs. how many times I’ve heard/seen then spawn, somewhere between 5% - 10% doesn’t seem entirely unrealistic. But then again, I could just be unlucky.