No, loot should not change with difficulty. It is in the name “difficulty“, you choose it if you want to challenge yourself.
Yes, droprates for Ember are absolutely unacceptable. I will go so far and say they are dogshit! When I clear a plagued area and open the chests again afterwards, if I find 1 Essence/Radiant Ember I may considder myself lucky.
The worst part is, you will get more loot by farming vendors in diffirent realms than by clearing maps. This is such a moronic design.
*looks at the clock* It is time to start the game, check Finley in my 3 realms, and then log out.