In the browser the content of  material "folder" is visible directly in the course if "inline on a course page" is configured.

In the mobile app the content is NOT visible. A work around is to set the folder to "On a separate page". Then in the app the folder-icon is visible and the content will be shown after a click at the folder-icon.

For testing:

1. Create a course and two folders to compare the behavior
2. Upload files to folders.
3. configure on folder with "inline on a course page" and the other with "On a separate page"
4. Open the app and go to the course.
5. The content of the first folder with "inline on a course page" is NOT visible but should be visible.

Workaround ideas

1. A work around is to set the folder to "On a separate page".

2, (Not very comfortable but in some cases if necessary copy the html-content of the filelist from the separate page to description of the folder and set "Display description on course page". This is only a trick and if you add files this will not be shown automatically in the course)