There is a Glitch between the Shade and returning to the room with Oryx. A guardian with full health can die randomly after Echo Oryx dies and shade is closing. I have heard it may have something to do with re-materializing in a rock pillar or another guardian. I am not completely sure the reason, but it has nothing to do with what the guardian does. There is no way to prevent this; it is random. Today is the first time it happened twice in a row.