The routes that you see when you book online may have no correlation to what is offered by the app as part of an SDC. Besides DEN, which may be a possibility (I’d have to look), none of the other cities can be routed on an IAH-LAX fare. Instead, what you’re seeing is the website constructing a fare by introducing a fare break at the transfer point.

The reason that matters is that the official rules restrict you to the routings for the underlying fare. That doesn’t mean that you won’t ever see something that’s prohibited — it wouldn’t surprise me at all for IAH-AUS-LAX to be offered by the app. It does mean that if the app doesn’t offer it, an agent might not choose to offer it either.

I have seen IAH-DEN-AUS and IAH-ORD-AUS offered; obviously, neither of those is a legal routing. It’s possible that the app just isn’t in a giving mood today.