I can join my friends world but they can't join mine.

There are no "world"s in the scope of the Mojang Web Services. If you are reporting a Java Edition / Bedrock Edition bug, please post your bug under those one of those projects. I would also recommend you provide more information, you can never specify too much information. Say what the error message was when you tried to join, or if you don't see an option to join their world, maybe their world is on invite only, and they haven't sent you an invite / their world is private. And please try to search for bugs that have already been reported that are similar to yours.