Hello,
Thanks for using our theme.
1/ Have you cleared all cache?
2/ Did you make custom changes in child theme files? Try to activate the parent theme and check add to cart behavior.
3/ Try to disable all the third-party plugins (including cache plugins), that do not come bundled with XStore https://xstore.helpscoutdocs.com/article/36-included-plugins , clear cache of browser, and check then.
Let us know the results.
Regards