It's not just a Garuda bug. It's any monster that teleports or vanishes from the arena for any reason. For reasons unbeknownst to me, your game fails to render the monster when they return to combat. Happens on ps3 a lot more often than on pc from what I hear, but both platforms have the bug. I've personally had it happen a few times on the Gaius Van Baelsar fight that has a lot of teleporting, but a second teleport normally fixes it. The worst thing about the bug is that it can happen to party members, not just monsters.