Thank you guys for responding! @Mark I think it is not very likely that I accidentally associated the DNG extension with AP myself. My normal operation is that I use my DAM as the hub in my workflow; in the DAM I select some images, click the AP shortcut in the DAM and the images then automagically open in AP. But you are right when you say that AP does not register itself as the default program. I uninstalled the program, wiped all traces, installed from scratch and the DNG associations remain untouched. So in the end I can not tell what happened here. My apologies for any inconvenience caused!
New user here, using Affinity Photo on a Windows 10 X64 box. After installation I noticed that AP had set itself to be the default application for .dng files. Of course I could undo this by going to the Windows file associations settings, but it is something that I feel should not be necessary. If it is necessary for some reason I think the installer should ask the user during installation if he wants the program to be the default application for certain extensions. On a slightly different but related note: I think it would be nice when the program would be listed in the Windows Control Panel\Programs\Default Programs\Set Default Programs hierarchy. That way the user could easily check for which extensions he wants the application to be the default. Thank you for listening,