How can Prompt be used to stop navigation on formik dirty state

I'm using formik in a reactjs project, and I want to use Prompt from react-router to open a notification before a user leaves and loses changes made to their submission.

I'd expected something like this to work:

<Prompt
    when={formik.dirty}
    message="You have unsaved changes. Are you sure you want to leave?"
/>

My formik block looks like this:

const formik = useFormik({
    initialValues: {
        <values>
    },
    enableReinitialize: true,
    validate,
    onSubmit: values => {
        <submit functional stuff>
    }
}); 

And my form is something like this:

<form id="myForm" onSubmit={formik.handleSubmit}>
    <div className="row">
        <div className="form-group">
            <label htmlFor="name">Name</label>
            <input
                id="name"
                type="text"
                onChange={formik.handleChange}
                value={formik.values.name}
                className="form-control"
                placeholder="Enter name"
                disabled={isDisabled}
            />
            {formik.errors.name ? <div className="text-danger">{formik.errors.name}</div> : null}
        </div>
        <div className="form-group">
            <label htmlFor="subject">Subject</label>
            <input
                id="subject"
                type="text"
                onChange={formik.handleChange}
                value={formik.values.subject}
                className="form-control"
                placeholder="Email subject"
                disabled={isDisabled}
            />
            {formik.errors.subject ? <div className="text-danger">{formik.errors.subject}</div> : null}
        </div>
    </div>
</form>

but it appears that formik.dirty is either not defined or it's not seen as true (despite making changes to the form).

How would I properly use the dirty prop to trigger the Prompt?

from How can Prompt be used to stop navigation on formik dirty state