I built an app that returns files in a document library. and provides links to those files using documentIDs. We migrated the site from 2013 to 2016 and now the links instead of opening the files, they take the user to the search results page and show the document there. The user has to click again to download the file.
This is the search query when I land on the page:
docid:SY54RF2SRMEY-32502834-8 OR dlcdocid:SY54RF2SRMEY-32502834-8
So it's doing a search for the document instead of just returning it.