Well if they were using Xbox Live I can't help 😞.
I'm not sure if this will work, but you can try this:
Go to Settings > Storage > World Templates
Select the Norse Mythology pack and delete it
Download it again from the Marketplace, create it and try to load the world again.
Are they joining by local network or by Xbox Live?
If they are joining by local network, have they done one of these things below?
Change username
Clear Minecraft data
Reinstall Minecraft
Ok, so it seems like it is not a bug, but will they ever make it like the Java Edition?
Portuguese
Intel i3 u380
Intel HD igpu
4Gb Ram
Could you make a copy of your world and see if the same happens?