We confirmed that in iPadOS 13.2 (17B84), released by Apple on October 29, there was an improvement with regard to the problem.
The following issues when using Clip Studio Paint for iPad in iPadOS 13(released September, 25, 2019) have been confirmed.
CLIP STUDIO PAINT for iPad 1.9.4
・Folders containing files from the file application cannot be dragged and dropped properly from the File app.
Please perform the following steps to move files and folders to Clip Studio Paint for iPad.
1. Open the Files app on your iPad
2. Press and hold the folder containing your work file, then select Compress from the context menu.
Note: We have received reports that folders on Dropbox cannot be compressed. Please copy such folders to your iCloud Drive or your local drive before compressing them.
3. Press and hold the compressed file, then select share. Select Copy to Clip Studio from the following menu.
4. The file will be unzipped in Clip Studio Paint, and the manage works window will appear.
When compressing a zip file on Windows or macOS, please make sure to encode the file in UTF-8.
If a zip file is not encoded in UTF-8, it may not be imported correctly.
Note: For other issues, please wait for further OS updates.