The default behavior of Magento's layered navigation is to filter products excluding taxes. This is a result of the inherent limitations within the default Magento logic, and it is not influenced by our Improved Layered Navigation extension.
Magento filters products based on indexed prices, which means that the filtering process relies on the indexed price values. However, prices that include taxes are not considered during the indexation process. As a result, Magento is unable to filter products by prices that include taxes, and our extension inherits this behavior on the frontend.