There is a bug that resets your all your cosplay upgrades (Sword, Defense, Hp, Mp) to 0.
EDIT: Actually, I tried defeating another yellow "echo original" and the bug didn't happen.
It happens when you defeat an Echo's "original cosplayer", that is, their yellow rare version, in Phoebe's zone. The equivalent of getting a figurine in the first zone, before the door.
By the way, I tested redefeating a yellow echo in the first zone (getting figurine again) and the bug didn't happen. Levels stayed the same, lost nothing.