Since I reported this issue I have figured out the solution! I think what I did was go onto the x-box website on my parent’s account, view my own account from my parent’s account, and change the permissions settings so that ‘join friends’ worlds’ is on. A simpler solution though is to log out of your x-box account on Minecraft so that the permissions of your x-box account don’t apply.
I have already tried every help page I can find, looked it up, and asked everyone I know, so I’m not sure if it is a true bug but it’s the only explanation left that I can think of.
Since I reported this issue I have figured out the solution! I think what I did was go onto the x-box website on my parent’s account, view my own account from my parent’s account, and change the permissions settings so that ‘join friends’ worlds’ is on. A simpler solution though is to log out of your x-box account on Minecraft so that the permissions of your x-box account don’t apply.