Today I tried to mirror a branch from my main personal document to a family document (that is shared with my wife) and I noticed that the mirror I created in the family document was not visible to her (in her it just showed [[Deleted Item]]). So I suspected that was because the original item was created in a document she doesnt have access to (I remember that is how it used to work). So I did the opposite, I moved the branch from my personal document to the family document, that worked well, however when I tried to create a mirror in my personal document from the original in my family document all I got was a link to a internal Id pasted in my original document.
So its odd that in one direction it worked, but in the reverse direction it didnt.
