Could this be related to the ability to pick entire folders in the file picker? 
https://social.blach.io/@textastic/112519168798881056
This was usually only available for iCloud Drive, but 3rd parties can now opt in into that as well. If that is the case, it should also work with Dropbox folders and OneDrive/Google Drive shouldn't work.
(Note, that Dropbox for iOS has a "Native Files.app integration" setting that allows to switch between a "classic" and a "new" integration.)