Issue is described in the Customer support under this post https://us.forums.blizzard.com/en/wow/t/whispers-defaulting-to-realid-chat-when-not-friends/228197/9
For some people while in game when I type in /w name it will for some reason switch to the realid chat color and appear in the desktop app social tab even when we’re not friends. Resetting chat, completely resetting cache, addons, and wtf folders did not fix this. Adding the person as a realId friend then removing did not fix this either. If the person whispers me, or if I right click their name in the guild community chat and select whisper, then the chat will be in the correct color/screen. This only occurs with certain characters of theirs and only when typing in /w name.
Edit: A I confirmed with both people and both of them have their battlenetId the same as their in character name. This might be the cause of the bug