Friend disappeared from friendlist.

Hello.
I have an xbox account. I added a brother who plays on PC. It was ok for a few days then my brother disappeared from my friends list. When trying to type in his nickname it doesn't search for anything. Only when I open the create team tab and type in his character nickname can I invite him to join me. He said that I am on his friends list and there is no problem. Also his whispers to me do not reach me.
Greetings