I am having a similar issue, I have the categories displayed on the left side but when I choose a sub category it's hidden under the product view...

In Odoo's eCommerce website, the visibility of product categories while viewing a specific product can be influenced by the website theme you are using. However, the default behavior in many themes is to display the product details without showing the category navigation.

If you want to customize this behavior and make sure that the product categories remain visible on the left while customers view a product, you may need to perform some customization. Here are some general steps you can take:

1. Enable Developer Mode:

    Go to your Odoo online instance.

    Enable Developer Mode. You can do this by adding ?debug=assets to the end of your website URL or by enabling it in the settings.

2. Locate the Theme:

    In Developer Mode, you can access the website editor. Go to the website dashboard and find the theme you are using.

3. Edit Theme Templates or Styles:

    Access the theme editor to modify templates or styles.

    Look for the template responsible for displaying a single product page.

4. Modify Template:

    Inside the template, you may need to add or modify the HTML and CSS to include the category navigation on the left.

6. Test and Save:

    Save your changes and test the behavior on the frontend.

Note:

    Customizing themes may require some technical knowledge of HTML, CSS, and the Odoo templating language.

    Be cautious while making changes and always test them on a staging environment before applying them to a live site.