quest

Flashy Flash

killing mobs sometimes doesnt count as the quest progress like "hit x enemies at the same time"even worse sometimes it will reduce the progress
and even i only hit a single mob it stll count as progress

wynd

it reduces the progress because "at the same time" really just means "within 1-2 seconds"
so if you hit something after that time passes, it resets as its not "at the same time" with the other ones
from what I remember all quests are completable, so unless there's a specific 100% reproductible way for this where you can prove you're hitting the required amount within 1s or smth I'd say its normal.