Xayah Passive decaying even while attacking
So basically, when you cast a spell with Xayah and get your 3 feathers, their decay will not refresh if you are attacking an enemy.
The timer only gets refreshed when you cast a new spell or if you attack towers or other units that don't consume your passive.
I don't think the passive is intended to decay during attacks considering the interaction with towers/traps/wards.