Overview:
When selecting Filters for the Vault Library, an Object Field cannot be selected without another Field being displayed along with it in the Filter.
Root Cause:
The second Object Field displayed for this Filter is set up as a Child Object of the first Object Field. This allows the user to select a related Child Object record for this Filter in order to narrow down the displayed Library results further if desired. In the below image, for example, Sub-Product is a Child Object of the Product Object, and both Objects are set up as Document Fields.
Solution:
To have these Object Fields displayed independently in the Library Filters, the Parent Object should have at least two Child Objects, where all three of these are Document Fields.
Related Documentation:
Vault Help Documentation: Configuring Vault Objects
Thank you