From the screenshot configuration :

1. Add a new file child of the "a" folder (it should be working)
2. Click on the folder "b"
3. Click on the "new file" button
4. On the dialog box, clock on "Browse..."
5. Select a file to add
6. (When you click on "OK", I don't know if there is a link with the issue, but notice that the app in the background flashed once)
7. Click on Ok on the dialog box, the newly added file does not appear in the tree, you need to refresh the "b" folder