This is honestly the first time I've ever heard this. Tell me more, this is the online version, correct? If you start a new save file, do you see the map at all? Or is the map disabled after doing something specific in-game?
I'm in the online version as I don't have a PC. I believe it may just be a network issue on my end. After experimenting a bit I've learned that it happens more often when I'm outside of the house and using my phones network and happens less often when I'm connected. Since I feel like I've figured out the issue I do have another question. What food are you supposed to give to Nullah to gain his trust? Or is it an unfinished encounter?
You need to find someone to feed to Nullah. His first casual dialogue option does give a hint by telling the player: "Seeing as how this area is rather close to Rett and Mitryll's cave, you can't help but to wonder if they ended up running into [Nullah] at some point."
After encountering Nullah and learning his name, you can head back into Rett's bedroom and there will be option of telling Rett about Nullah using his own hunting grounds. This will cause Rett to go confront the yellow dragon, and thus cause him to get devoured. Try it out yourself and see how the scene plays out!