Check it out, doing the raid about to kill Calus with a group and this happens!
https://www.youtube.com/watch?v=z0aHarsuDjY
-
Had that happen on a Prestige run after we got his health all the way down on second plate. My understanding of the glitch is that if you keep shooting him after his health drops to zero it may cause this bug. I noticed while watching the video that you had done damage to him even with his health at zero just before his shield went up, so that may be the issue.