Having this same issue on Xbox One, and was directed to this bug by Microsoft Support.
If I sign in on my account (realm owner), only one other player can join. A third player (second "friend") cannot join, and is given the "Server Full" message.
However, if I sign out, two other players can join the realm, and I am able to join as the third. A fourth player gets the "Server Full" message.
All four of us were able to play on the same Xbox One with the "2 friend" Realm prior to this.
This bug is NOT resolved.
Having this same issue on Xbox One, and was directed to this bug by Microsoft Support.
If I sign in on my account (realm owner), only one other player can join. A third player (second "friend") cannot join, and is given the "Server Full" message.
However, if I sign out, two other players can join the realm, and I am able to join as the third. A fourth player gets the "Server Full" message.
All four of us were able to play on the same Xbox One with the "2 friend" Realm prior to this.