I have this same issue with my character. Last night, I tried loading my daughters world (loaded fine) then I added my OTHER daughters’ character (loaded fine) and finally loaded my character in the third spot and got the “Building Terrain” screen. So it appears that this issue is with my CHARACTER as opposed to the game itself.
Actually, mine is the same but I tried to load my OTHER daughters game first and it DOES load.
So let me explain. When I load my first daughters game (her name is Gabby) and then load my character, the screen is stuck on “Building Terrain.” However, I tried this last night. I loaded my daughters world (loaded fine) and THEN I used my controller (it’s a classic GameCube controller) to lose my other daughters character (her name is Athena) and the game loaded fine (split screen). So THEN I tried loading my character again on the 3rd screen and the “Building Terrain” came up again.
So it APPEARS to be an issue with my character, since both of my kids can still load their characters fine. Whether I am second or third player, my character gets stuck on the same screen.
Update: Gave up. Tired of trying to figure it out. Made a new account JUST so I could play Minecraft with my kids. Also - you cannot play multiplayer without a Nintendo account...? I don’t get that, especially since my kids don’t go online with other people...
Anyway, lost a few building materials, nothing much. But if this happens again I will report it... again... and hope the MOJANG finds out why this is happening. I didn’t lose a lot, but I’ve read some people online lose everything. I was devastated to think I would no longer be able to play on my kids’ world and have to start completely over.