Mass evolving for XP is a common thing in Pokemon Go, but the latest bug has made evolving more than one Pokemon at a time difficult.
Niantic twitted about the new bug in Pokemon Go, this bug causes the list of Pokemon to scroll back to the top whenever a Pokemon is evolved or transferred to the professor. So to mass evolve, a player has to scroll down after every Pokemon they evolve.
Most trainer mass evolves to gain Double XP using Lucky Egg 30 minute window, but due to this bug, Lucky Egg will be wasted if activated to mass evolve.
We’re aware of the bug which causes you to return to the top of your Pokémon list every time you make a transfer to the Professor or rename a Pokémon. Check back with our known issues page for the latest updates: https://t.co/TKhSg9bUUB.
— Niantic Support (@NianticHelp) December 22, 2017
At least this bug isn’t a gamebreaker and it seems like Pokemon Go is already working on a fix. With Niantic working on Holiday event this bug could take few days until it gets fixed.