Steps to reproduce the issue

Created a plugin to add extra field to the Category edit page as I cannot use 'com_fields' Custom Fields for various reasons. Code as below for this. Field renders as expected.

categoryextension.xml:

<?xml version="1.0" encoding="UTF-8"?>
<form>
    <fields name="params">
        <fieldset name="category-icon" label="Icon">
            <field name="icon" type="text" default="icon-round" label="Category Icon" description="Enter the category icon" size="100" />
        </fieldset>
    </fields>
</form>

categoryextension.php

<?php
// no direct access
defined('_JEXEC') or die('Restricted access');

class plgContentCategoryExtension extends JPlugin
{

    /**
     * Load the language file on instantiation.
     *
     * @var boolean
     * @since 3.1
     */

    protected $autoloadLanguage = true;

    public function onContentPrepareForm($form, $data)
    {

        $app = JFactory::getApplication();
        $option = $app->input->get('option');
        $view = $app->input->get('view');

        switch ($option) {

            case 'com_categories':

                switch ($view) {

                    case 'category':

                        if ($app->isAdmin()) {

                            JForm::addFormPath(__DIR__ . '/forms');

                            $form->loadFile('categoryextension', false);

                        }

                        return true;
                }

            return true;
        }

        return true;

    }

}

Expected result

Custom field is rendered. Data is saved to the database.

Actual result

Custom field is rendered. Data is NOT saved to the database.

System information (as much as possible)

3.9.8

Additional comments

Here's a very old blog post about this exact issue, in Joomla 2.5 and 3.0, but the code in com_categories edit.php has changed a lot since then.... https://techjoomla.com/developers-blogs/joomla-development/adding-custom-fields-to-joomla-categories-in-joomla-25