I had received a new friend request after finishing a mythic+ dungeon and upon accepting all my friends were deleted besides the new person. I have tried all available steps (cache/reinstall/password reset) and none have seemed to work.
I was still being given in game notifications when my friends joined groups but I could not whisper any of them or see it in the quick join tab.
Using the beta client for battle.net if I click on Chats and Groups I can see all my friends and our message history. So there is some weird bug happening.
Update: Whatever was going on seems to have been resolved and my friends are back.