If you can't stand to read another post about Anhinga, talk amongst yourselves.
I think I've worked out what the problem is. I wore the sweater yesterday afternoon and evening and it grew quite a lot, which revealed that the seam between the flap and the left side showed up as quite short. The 'seam' between the flap and right side is false - it's created by picking up stutches on the other side - but the one on the left creates some of the tension that makes the construction work. This wouldn't be a good pattern to do in the round, except maybe for the sleeves, because it relies on the construction to make it do its thing.
That was the first seam I did and had quite a lot of trouble with because sewing with Rowan Bamboo Tape was a new skill (can you hear my clenched teeth?); I probably made it too tight. I’ve wet the jumper again but when it’s dry I’ll unpick that seam and sew it up again more loosely. That will stretch that edge a little which in turn will pull up the lower edge. She’s a clever girl, that Norah Gaughan.