That intersection is where the waistband intersects perfectly with the angle of the already-sewn dart (pressed towards CF). The acute angle of the corner, the top of the dart and the angled "side" seam will all serendipitously meet at exactly the same point.
Normally when you have to sew an inset into an existing corner the corner is capable of being pulled open (by clipping into the reinforced point). I'm reluctant to do that here, as the topstitched SA of the "side" seam continues up and towards CF. I think clipping into it would create a big structural weakness at that point.
I'm seriously considering pressing all the SAs on the waistband to the inside, and applying it by hand.
Does anyone out there have advice for me?