My kids 8 and 6 start a new map. 

as multiplayer is on, someone just logged  in and destroy all what they have built with tnt.

so here is a list of bugs I see: (3 is the most important)

1)I was not signed in to my Microsoft account. so why my local map is even public?

2)I  have only 1 online friend in Microsoft account, so this guy is sure not a "friend of a friend". how should he got in?

3) why new map is open by default, make more sense to have it closed, and open to network if you want.

4) if all above not considered a bug for you, and it per design. why I cant block this dude or even report this behavior?