I have played on my Realm for over a month and me and my friends have created a Nether tunnel network with portals, but as of updating the app and Realm to 1.13.0, out Nether portals have become unreliable. I entered the main Nether hub and went to a portal 600 blocks away and it should've teleported me about 4000 blocks away from the hub but instead it just teleported me to a similar 600 blocks away from the main portal in the overworld, and it didn't even create a new portal there, I just fell into an ocean. Even after breaking and resetting the portals they still unlink from each other and it's quite frustrating. I hope this gets fixed in 1.13.1 or 1.13.2 soon.