REST API add records to data group (v14.1.)

Looks like I should have paid more attention to the error.

I noticed it was complaining about trying to change the type, so adding the type:string to the JSON body resolved my issue.

{
    "type":"string",
    "records": [
        {
            "name": "test",
            "data": "stuff"
        },
        {
            "name": "testing",
            "data": "more stuff"
        }
    ]
}