Custom fields: can’t change field ID

Hi! Thanks for a useful plugin. It’s mostly been pretty straightforward to use, but I have come across a bug with custom fields: changes to the field ID input area don’t overwrite the default field ID.

I can type in the Field ID field and save my custom values, and the text that I entered is still there in the text box when I reload the page, but that doesn’t change the actual field ID (see the snippet of source code below). This is evident both in the custom field group source code (see the snippet below) and in the shortcode given on the custom template where I’m using this field group.

<div class="wp-cpt-field-row field-form" id="field_j32nvx">
    <div class="field-form-sx">
        <label>Field name            <input type="text" name="field_settings[fields][field_j32nvx][name]" placeholder="Field name" value="Name">
        </label>
    </div>
    <div class="field-form-dx">
        <label>Field id            <input type="text" name="field_settings[fields][field_j32nvx][id]" placeholder="Field id" value="website">