You seem to have the correct approach here. Can you check check your form document in Photon (in the medic db, there should be a doc with "_id": "form:your_form_name) and see if it contains the expected hidden_fields array as a root level property? If so, then we know the config change has actually made it to the server. Then, if you are using an offline user to submit reports, be sure to “Sync now” for that user to make sure they have the latest config before submitting a new report.
You could also try using the instance::tag = hidden configuration in the form xlsx itself (as mentioned in another thread) to hide the fields when on the Reports tab. That does provide more fine-grained control over which fields are hidden, but really either should still work fine.