Just had this happen, 3rd crystal (the lowest one) won't trigger even when shot with large volumes of small arms fire, heavy weapons, grenades or even supers. There is no other way inside the boss room than this door, and no other way to open said door, meaning this is game breaking for Onslaught. Getting this bug just ends your run. I read plenty of people getting this on wave 50 on Legend mode, but I just had it on wave 30 on Normal so it can probably happen anywhere a boss door is involved. Several threads on Reddit document this bug as far back as 20 days ago.