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.
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Dug up an old reddit post from over a year ago. If a titan places a barricade on either plate it will make the crystal on that side immune. BUNGIE fix this garbage its more annoying now with the Last Word quest.