List form with JSON issue

Is there a criteria where the the field you've selected to view in the form header won't display? In all of the examples it works with a 'Status' field, but when I try to change to any other field, it won't work. I thought it was fields that had spaces in them, but I've just tried with a simple field named ID (it's the title field) and it still doesn't display. I'm so confused why it would display some fields and not others?

            "children": [
                {
                    "elmType": "div",
                    "txtContent": {
                        "operator": "+",
                        "operands": [
                            "Job Number:  ",
                            "[$ID]"
                        ]
                    },