Was doing Legend Seraph's Shield with a friend and we got soft locked due to the issue in the title. I think that the mission objective might not have changed to acquire the keycard iirc, but I may be wrong about that. Restarting the mission fixed it.
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Usually if it bugs out, its a early checkpoint update that was missed. Bungie are aware of it but as we're less than 2 weeks away from Lightfall its unlikely to be fixed.