@Lachica crudely drawn, the thread path I mean, is almost like two opposing runs of backstitch
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If I run that down the back just towards the point, it should take a little of the tension off the fabric at the front.
After fiddling around with it, I probably also need to pleat below the point slightly, which will bring the two seam allowances together. With a 5mm seam allowance either side of the cut, that leaves 10mm of material width bunched up under the infill piece, which is probably not helping matters. If I pull that bunch together into a pleat immediately below the apex, then do that curlicuetype stitch above, hopefully that will fix it up nicely.
As for covering it, more often than not, I tuck shirts and tees in. That said, the belt will cover the waistband, so there'll only be the point visible. As you say, probably not even noticeable... but if I can bull it up with a few extra stitches, it's worth an extra couple of minutes of faff.