Hello @Jason13 and welcome to the community ,
First of all , you need to know that , for settlement purposes , Unibet ( Kambi ) takes it's official results from premierleague.com not from flashscore.
The official stats do not indicate any assist from Nicolas Pepe during the match , so therefor , this is the reason why the bet is settled this way .
I've watched that last goal being scored by Arsenal , and it seems like even tho Nicolas Pepe sends the ball to Aubameyan , it was deviated by Masuaku .
flashscore.com is not anymore a trustfull provider of sports informations . are others than are better than this : sofascore.com for example
anyway when you bet selections like this is much better to read T&C because for Champions League might be other provider than Premier League and so on.
anyway probably at Unibet the settlement it was made manually since it is from Premier League official website with no automatically feed .only mistake it could happen is from the Premier League website