No business class seats are available to purchase w/ cash from JNB > DOH > SEA on 8/18. I can buy business class separately from JNB to DOH then from DOH to SEA.

It's especially odd to me because I can buy the full itinerary in business for two earlier flights that day, but not for the latest flight, which happens to be the time that works for me.

I'm assuming this has to do with the fare codes and/or married segment logic but don't fully grasp what's happening here. I'd greatly appreciate further insight into this.