Hidden fields do their job too well
"Hidden" fields are meant to be hidden on profiles, and hidden (but showable) by default on FormBuilder.
It turns out they're hidden on all QuickForm pages.
Attached are two screenshots of a contribution with a hidden custom field "Outreach Method", which has a value of "email". Please ignore the other custom fields - I didn't make this db
Now in the longer term we can assume these pages will be replaced by FB and it will be a moot point, but for now, it seems like the correct behavior for hidden fields should be "visible on the back end". There are certainly instances where that wouldn't be desirable, but for now I think we need to accommodate the most common use case. Which this will be until we remove the unfortunate overlap with "reserved" custom fields.