FlexSlider on Woocommerce's product page: Prevent vertical swipe while swiping horizontally

in WooCommerce, with my Storefront child theme, I have an issue with FlexSlider on iPhones (iOS): when swiping left or right the product-image carousel, there is some level of vertical movement, which I don't want.

You can see the bug in this video : https://photos.app.goo.gl/QaabMFwhskrEeys88

I have this in functions.php:

/*PRODUCT PAGE FlexSlider Options*/
add_filter( 'woocommerce_single_product_carousel_options', 'filter_single_product_carousel_options' );
function filter_single_product_carousel_options( $options ) {
    if ( wp_is_mobile() ) {
        $options['controlNav'] = true; // Option 'thumbnails' by default
        $options['direction'] = "horizontal";
        $options['slideshow'] = false; // Already "false" by default
    }
    return $options;
}

Any idea how to prevent this vertical rebound?

from FlexSlider on Woocommerce's product page: Prevent vertical swipe while swiping horizontally