I had this issue in 10.11.6 and unfortunately the upgrade to 10.15.6 did not fix it so I'm posting this here. Whenever I open a folder, that's in the Dock, in Finder (for example using "Open in Finder" in grid view or the equivalent in any other view) it opens the folder in Preview instead, and Preview shows all images/documents in that folder (and apparently subfolders). Does anyone have any idea what's going on and how I can resolve it?