Flows 4.0 do currently not allow init-value to use a boolean data variable

{
    "type": "OptIn",
    "name": "i_am_boolean",
    "label": "A Boolean",
    "visible": "${data.i_am_boolean}"
}

with

"data": {
    "i_am_boolean": {
        "type": "boolean",
        "__example__": true
    }
}

shows an error:

Data model schema is invalid for dynamic data "${data.i_am_boolean}" defined for property: init-value of component: OptIn in screen: MAIN, expected the type to be string.

How are we supposed to initially check an OptIn field ?