Steps to Reproduce:
1. Go to Content > Documents and Media > Document Types > Create a new document type
2. Add a Date field to the Main Metadata Fields
3. Go to Documents and Media > Click on + > Multiple Documents
4. Select any document to upload, select the custom document type, choose a date for the DDM date field, and click save
5. Edit the document
The date chosen when uploading is saved to the document.
The date picker is empty.