After constant beatings from the waif, Arya's face has never shown a bruise or a scratch.
There is a Fight Club theory that Arya and the waif are
the same person. The fighting represented a mental battle inside Arya between "old Arya" and "no one / the waif". The first "fight" in "Fight Club" is Edward Norton beating himself up in a parking lot. I don't think Norton began showing bruising until he started fighting real people.
Or, I would think that always having to look at Arya with a bruised face would be unpleasant. And, it'd be a pain for wardrobe.
Why don't they show bruising on Arya?
(1) is Arya fighting with a real person, the waif?
(2) is she fighting with her inner Tyler Durden?