It sounds like you’ve encountered a change in behavior with the Add To Cart button in your AbanteCart setup. Since you've done a fresh install and tested it with a clean HTML file, it seems that the functionality might be tied to the settings or perhaps a recent update or change in your theme or extensions.

Here are a few suggestions to help troubleshoot the issue:

Check Settings: Go through the settings related to the cart behavior in the AbanteCart admin panel. There might be an option that controls how the Add To Cart button functions.

Inspect Extensions: If you have any extensions or custom modifications installed, try disabling them one by one to see if they affect the behavior of the Add To Cart button.

Theme Compatibility: Ensure that the theme you are using is compatible with your current version of AbanteCart. Sometimes, themes can override default behaviors.

JavaScript Errors: Open your browser’s developer tools and check for any JavaScript errors that might be preventing the cart from functioning correctly.

Documentation: Review the AbanteCart documentation or community forums for any recent changes regarding cart behavior or settings.

If you find that everything is set correctly and the issue persists, you might want to reach out to the AbanteCart community for further assistance, as there could be others who have experienced a similar issue.