The fairy was disappearing long before 5.0 but I only noticed it in the MSQ raids. Her pathing has gotten erratic which causes you to become to far away in some cases and she despawns. The best fix would be to have her warp to you and auto follow if she gets to far away. Or make her insta-cast like the summoner pets so we can have as much uptime as possible. We are the only healer where part of out toolkit might go randomly missing.