First of all, the -50 delta for mythic is stupid. That said, I've finished the entire step of this quest except this one secret strangelet. The tank just vanishes.. anytime we had a good run going, the tank would jsut disappear making it unable to complete that strangelet. We tried attacking the bosses in different orders to see if we could ge tthe tank to stick around, but everytime we had a good run going with time on our side, the tank would just vanish causing you to not be able to complete the encounter. Playing on xbox series X