SharePoint 2013 Workflow: Retrieving hidden field value always null

I have a SharePoint 2013 workflow in a Visual Studio project.

I use a LookUpSPListItem activity together with a GetDynamicValueProperties activity to get field values from a list.

All field values return correctly except from a hidden field in the list that is of type string; it returns as null.

When I set the field to Hidden="FALSE" in the list schema and redeploy, it works fine. Set it back to Hidden="TRUE" and the workflow fails to retrieve the value again.

Why is this happening? Can workflows not retrieve values from hidden fields? Is there a workaround?