Overview:
The user is not able to filter or report on an object-type Document field that is based on a Join Object.
Root Cause:
This is a known limitation within Vault.
Solution:
This is a known limitation when using simple Join Objects as object-type Document fields in Vault. The simple Join Object always displays in Search filters and reporting as the Join Object instead of the object-type Document field.
Related Documentation:
Vault Object Documentation: About Object Relationships
Thank you