First, native Pre-Fill is never supported on non-Marketo LPs (you must use my custom JS solution for that). So what you’re seeing cannot be “Pre-Fill” (using Marketo terminology) in any case.
The problem is in your form config, you have the default value set to “Opt In”. This is apparent from the form descriptor:
[{
"Id": 231670,
"Name": "Text_Messaging__c",
"Datatype": "checkbox",
"Maxlength": 255,
"PicklistValues": [{
"label": "Yes, I would like to receive text messages",
"value": "Opt In"
}],
"InputLabel": "",
"InputInitialValue": "Opt In",
"InputSourceChannel": "constant",
"InputSourceSelector": "",
"VisibleRows": 4,
"ProfilingFieldNumber": 0,
"ValidationMessage": "This field is required.",
"DisablePrefill": true
}],
See where it says “InputInitialValue": “Opt In"? That’s the way you have it set in Form Editor. If you can’t see this setting now, it’s possible that if you switched the form field type to checkbox some old setting is still sticking around. If delete and re-add the field, you’ll be back to the default state.