I started the final Arbiter mission of 100M damage on a single difficulty today. I started on Nightmare clan boss. I used 3 keys and got my 39M damage that I needed, with the intention of then switching to Ultra-Nightmare clan boss to complete the Arbiter mission of 100M damage.

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After using 3 keys, this is the mission progress that was shown:

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Ok. All well and good. Now, what I was expecting, and what is not explained anywhere that I can find, is that once you switch to Ultra-Nightmare, the damage you inflict on that difficulty is not counted towards this mission. I was expecting that once I had dealt over 39M damage to the Ultra-Nightmare clan boss, this mission progress bar would continue to rise again, because again, it is not stated anywhere that you can not switch the difficulty once you've started on a specific difficulty. It only states you must deal 100M damage to the clan boss on Nighmare OR Ultra-Nightmare on a single day.

Here is the damage I dealt to the Ultra-Nightmare clan boss, after spending way too many keys to do it.

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Now, according to this screenshot, I have done over 100M damage to the Ultra Nightmare clan boss on a single day, however, when I went to check the progress status on my Arbiter mission, it still showed the above number of 39M. If this is not a bug then it is not properly explained in the mission description and should be fixed. I hope this information prevents others from wasting as many keys as I did to get this mission complete. Because of this issue, I had to go BACK to NM difficulty and spend 4 more keys to complete the mission that should not have been needed. The additional damage done to the NM clan boss was worth nothing other than mission progress. Thank you for taking the time to read.

~Gamercop