In September the flight itself doesn't run daily, so it makes sense that business class doesn't show up.
In Auguest, the flight is running (available for sale in Economy and Premium Economy on every Tues-Wed, and available for business on every Tues-Wed except the ones I mentioned). Availability on airfrance's own website is the same.
I guess there's no way to know why those 2 dates aren't available, will just wait closer in if it opens up.