The ball was there to be won from a heavy touch so no complaints there. In real time it looked like Stephens got the touch on the ball, the replays are (somehow) less conclusive. I've seen it from a few angles now and still can't work it out, looks to me like Dale did play the ball back against Boufal and this is what tripped him. Considering how close Swarbrick was we'll have to give him the benefit of the doubt I suppose.
There's an awful lot more analysis on here of whether that was a foul, than why the ref didn't blow for Knockaert's handball OR Izquierdo's clear foul, in the build up to OUR goal!