The rationale behind the change (" if a battle is completed too quickly or if the enemies are too easy, rewards will be reduced proportionally.") is that players are going to blow through level 1 explores much faster than level 12. And that’s true. The fact is though that the rewards from a level 12 are still capable of being nerfed – not because the enemy level is too low, but because the fight can be completed too fast. With level 1 difficulty, both boxes are checked. With level 12, only the “too quickly” box gets checked. I posted about it here actually, 4 days ago:
My argument with Fourdottwoone is because of the statement made that “If you play stage 12 explore you get full rewards if you play for about a minute.”
In my posts which I just linked to, I argue that I should get full rewards no matter the length, because I’m not going to sit in a level 12 explore for longer than I have to. If I can beat it in 30 seconds, I will.
Which brings me to the main point: level 1 is just better, because vault keys and epic vault keys can’t be reduced any further, so you’ll always get more keys doing level 1 than you will level 12.
I could see an argument for doing level 12 IF the rewards were never nerfed regardless of duration of fight, because then you get more gold, more rewards, and more medals… but until then, it’s just a waste of time to sit around in a level 12 for exactly a minute just to get “max” rewards. You’re better off trying to fit as many level 12 fights in as you can. It’s better to do 2 level 12s per minute than 1, for instance. And by that logic, it’s better to do 4 level 1s per minute than 2 level 12s, since keys are what you should be after.