A bug introduced in the last (3?) updates ago:

I have 2 devices, both on the same wifi network. Both with the same email address.
The 1 device I am like level 30 on this device.

The other device is my main (real) gameplay. I am level 66 on this device.

So, I just noticed, that the level 30 device ups the arena and (Krampus) boss energy 3x faster than the main level 60 device!

This only happens when both devices are connected to the wifi, and running.
Previous to (3 patches ago), both devices/characters worked correctly, and upped energy at the same speed.
Now, I have to log out of 1 device, disconnect it from the wifi, and then launch K&D on my other device. This way, they both level up energy the same speed.
But, this is a PITA.