When a record is being created with multiple Document Library fields, choosing a file for any of the fields will result in only choosing a document for the last field.
1.) Create a Data Definition with 3 Document Library fields
2.) Create a List using the new Data Definition
3.) Upload a file to the Documents and Media portlet
4.) Begin adding a Record to the new List
5.) Try choosing the uploaded file for each of the Document Library fields
Only the last field will have a file chosen for it.