The witches ritual on titan is bugged. The shield will go down on both Crystal's but only 1 crystal will be able to be shot. The other is immune even after the shield drops. This has happened multiple times already.
Can confirm that this happens on mars too, just myself (hunter) and another hunter were trying to do it fir Last Word. Crystal was immune even after the shield went down. Please Bungie, this is not the kind of thing you need getting to the press.