This is a general issue with the file picker. It can be replicated anywhere there is a filemanager element is used, although how a page reacts after this may differ.

Replication steps:

1. Log in as teacher/admin
2. Navigate to a course
3. Create a Folder resource
4. Add some files normally
5. Click to Add... files
6. Select the "Upload a file" repository
7. Drag a file onto the Attachment field
8. Click "Upload this file" - The file manager appears empty with a red dashed outline.
9. Click Save changes - The files are still there, including the new one