Can a report return results for a Hidden Document Field?
No. If a document field's Default Security is set to Hidden, then the report cannot access the field and will return an inaccessible field error message that references the hidden field. If a field is set to Hidden for some users, then the error message will appear when one of those users tries to run the report.
Additionally, if a report has a formula field that uses a hidden field, the user may receive a server error when trying to run the report. The server error occurs because the hidden field prevents the formula field from completely processing at run-time. If the user clicks on the information icon in the server error message bar, they may see an additional unknown field error message referencing the hidden field.
To remedy this issue, an admin should update the Default Security permissions on the field to Read-Only or update the permissions on the field appropriately for the affected user groups.
Vault Help Documentation: Types of Field-Level Security