Ok so I gave you the wrong date. It was Aug 24. I think I was thinking Sept because that's when you said you wanted to fly, so it was in my head. But I had searched for Aug, since, as you say most of Sept was unavailable.

I also find that sometimes when searching on the app and selecting business/first, it will show you economy prices anyway below the dates. But if you select a date it will show you business / first availability if it exists, despite the heading showing the economy price. So you need to check every single day instead of just looking at the headings. It wasn't doing that a week or so ago.