It is working as designed in system that product classes are not included during the import process, customer. I can give you insights about importing product in QuickBooks Online (QBO).

When you reimport items, our system is configured to override existing information based on the data contained in the import template, which currently does not include classes. As a result, any previously assigned class will automatically disappear from the overridden items. To update class information, you will need to manually edit and add the class to each product after its import.

If your main requirement is to update stock levels rather than product information, you can make direct inventory adjustments instead of reimporting the entire product list. This method will allow you to update your stocks without affecting the class assignments for your products.

For a more detailed understanding of the types of information you can import into QuickBooks Online, please refer to the article: Common Questions About Importing Data to QBO.

Additionally, I've included this article that provides a helpful guide on how to categorize your products and services effectively in QBO. This will assist you in maintaining a well-organized and structured inventory: Group products and services into different categories in QBO.

We're committed to assisting you with a smooth import of your products into QBO. If you have any further questions or need additional support, please don't hesitate to respond to this message. We're here to help and will get back to you promptly.