While hosting a multiplayer world on Windows 10 (may be impacting other platforms), friends that were added on Xbox live after the session has started are unable to join and receive an error stating "This worlds multiplayer setting is set to friends only. You must be friends with the host of the world to join".

You are able to see on the Xbox console companion, from both users, that the host and the other players are friends. You can see the world as joinable from the Xbox interface and within the friends tab in Minecraft, however joining fails and results in the error that has been mentioned. 

Edit: 

Steps to Reproduce:
1. Start a world on Windows 10 with the multiplayer settings set to "Friends Only".
2. The host adds a player as a friend on xbox live after they have loaded into the world.
3. Ask that player to join from the friends tab on Minecraft.

Observed Results:
**The newly added friend will be unable to join your world after selecting it from the friends tab. They will instead see a screen stating that you must be friends with the host to join. This behavior will persist for anywhere between 20 minutes to 1 hour. this wait is plenty enough time for the player to lose interest in joining and ruin the experience, therefore waiting would not be an appropriate workaround. 

Expected Results:
After the two players are friends on xbox live and the world becomes visible on the friends tab of minecraft, that friend should then be able to join the world. 

Note: This bug impacts only new friends. Players already on the hosts friends list before the multiplayer session has started do not seem to be impacted.