The attribute appeared to be created correctly in the eav_attribute database table, but the frontend value was not being saved to the database when the customer registered.

The attribute was a checkbox, which has a boolean true/false value. As I set the checkbox to be checked by default, my mistake was simply not to set a default value which meant that no value for the custom attribute was being passed via the registration form to the backend customer registration / creation process.

    <div class="field gj_custom_boolean_attribute">
        <label for="gj_custom_boolean_attribute" class="label"><span><?php /* @escapeNotVerified */
                echo __('Customer Custom Boolean Attribute') ?></span>
        </label>
        <div class="control">
            <input type="checkbox" checked name="gj_custom_boolean_attribute" id="gj_custom_boolean_attribute" title="<?php /* @escapeNotVerified */
            echo __('Customer Custom Boolean Attribute') ?>" class="input-text" autocomplete="off" value="1">
        </div>
    </div>

In case anyone else is trying to create a custom checkbox (boolean) attribute and experiencing the same mind numbingly annoying problem here is a module with a demonstration of two custom Magento 2 attributes – checkbox and text input that demonstrates the correct working code.

https://github.com/gaiterjones/Magento2_Gaiterjones_CustomerRegistration

The module creates two attributes:

• gj_custom_boolean_attribute – a sample boolean checkbox attribute
• gj_custom_text_attribute – a sample text attribute

The additional registration html is in the additionalinfocustomer.phtml template file. Note here that the checkbox requires a default value

value="1"