SYMPTOM:
Option of "Expand Page-by" is available even for a multi-layout document with none of the layouts having a grouping section in Strategy Web 9.x.
STEPS TO REPRODUCE:

CAUSE:
This is working as designed. In Strategy Web, we cannot tell if there is any page-by in the other layout when a document has more than one layout. And it needs the communication to Intelligence Server to fetch the definition of the layout that the user currently choose. Therefore, the option is always available for multi-layouts document.
ACTION:
There is no action required as this is working as designed.