Unsure if others have discovered this, but since the servers came back up, I found [some], but not all, shared estate settings had gone wonky.

Messages saying this person was not on my friendslist (while staring at them on said list);
Messaging saying this person does not exist (also staring at name on friendlist).

To fix it, I just removed the person and set them a new friend request, adding them back to the shared estate.

Be sure to check your shared settings if you share a house with friends.