Automatic Platform Framing has been changed in X13 to follow the Platform Framing style of building more closely than mixing together both platform and balloon framing styles. If an open below section is partially along an edge of the platform, then we'll have the rim joist extend the entire length of this edge. This is where the behavior has changed from prior versions, where we used to always balloon frame along any open below sections that were inline with a platform edge.
If the open below section spans the entire length of the platform edge, then we still do balloon framing, if the "Allow Automatic Balloon Framing" checkbox is enabled in the Build Framing Dialog. If the checkbox is disabled, then we'll insert rim joists in this section to match with the rest of the platform framing style.
With "Allow Automatic Balloon Framing" On:
With "Allow Automatic Balloon Framing" Off:
If you wish to do balloon framing for only a section of the wall, this can still be achieved with the Platform Intersections controls within the Wall Specification, where you can set specific walls to balloon framing through the ceiling or floor. On one of the walls going alongside the open below, you can break the wall so a section of it is only going across the open below, set this section to "Balloon Through Floor Below", and you'll get the balloon framing along that section of the platform.
If you would like the program to work differently to this new behavior, then please submit a feature request. Hope that helps!