In the latest update of the Adobe scanner the documents are automatically searched and recognized. However, often this function does recognize only part of the document, so that the scan is not the complete document. As far as I can see there is no manual function for correcting the frame of the scan area and there is no possibility to cancel the automatic document search.
How to solve this problem?
The document detection feature has been added to simplify your search of finding the only documents from your phone's picture library.
However, the document correction you are referring above has always been there when even with the earlier version.
Adobe scan app automatically corrects the frame of the captured document. Not with all the document, but few those have not been captured correctly or not symmetric.
Even if you ignore the option of choosing the document from auto detect documents menu and select them from "Show All Photos" (as shown in picture below) then manual correction will do the same and correct the frame accordingly.
As "Document Detection" is the latest feature has been added by default. There in'st a way to turn it off manually.