Hmm, I just double-checked and I have no problems using the hidden_fields attribute in the form properties to hide fields on the Report page. (Silly question, but I have done this before, so seems worth checking: you are using formname.properties.json and not formname_properties.json right?)
As you may have noticed in the default forms we tend to use the instance::tag = hidden in the actual xmlx itself more than the hidden_fields in the properties. Both should work (and can even be used together), but the instance::tag provides more fine-grained control of which fields get hidden.