Hi!
I got an issue with which mana to spend for the new elementals in combination with Kavaero, Mind-Bitten and Esper origins + Badgermole Cub.
Let's say that I want to cast Kavaero with 2 black and 2 blue mana to have him as a copy of Deceit but I also have Summon: Esper Maduin on the board that has just given me the 2 green mana, the game would absolutely not let me use 2 black and 2 blue mana and will always use the 2 green 1 blue 1 black if I have anything in hand that's not green (tapping lands with the right mana I want won't change anything to the issue). The same thing would happen with Badgermole Cub and his earthbend ability + the fact that you get 1 extra green, the game would always only let you use the green mana if you have no green cards in your hand even if you want to cast elementals for their trigger ability.
I'm on PC.
I hope I explained the issue in a clear enough way and I wish you a very good day!