I am having the same issue on my Realm. I've attempted with and without Moss, in various biomes, even with a different player placing the egg on the Realm, each time the same thing happens as in your video.
Is there anything I can do or share to help understand the problem?
Hi, I think I have discovered what is causing the issue. The Sniffer's that are spawning are actually invisible! In the video you can see the egg disappear and nothing appear to spawn. I then attack and see that I hit something and it died. You can see the second egg spawn but I then saved, closed and disabled the "Texture Update Pack", and when I re-entered the Sniffer from the second egg was now visible.