wuspum

https://i.ibb.co/HHqnXSz/Screenshot-1.png

The thing here is that something must’ve changed between these two woocommerce versions that “broke / changed” the functionality. Checking the cart worked well in custom endpoints even when logged in. At least using this function.

function check_prerequisites() {
    
    // var_dump(WC()->cart);
    
	if ( defined( 'WC_ABSPATH' ) ) {
		// WC 3.6+ - Cart and other frontend functions are not included for REST requests.
		include_once WC_ABSPATH . 'includes/wc-cart-functions.php';
		include_once WC_ABSPATH . 'includes/wc-notice-functions.php';
                include_once WC_ABSPATH . 'includes/wc-template-hooks.php';
	}

	if ( null === WC()->session ) {
		$session_class = apply_filters( 'woocommerce_session_handler', 'WC_Session_Handler' );

		WC()->session = new $session_class();
		WC()->session->init();
	}

	if ( null === WC()->customer ) {
		WC()->customer = new WC_Customer( get_current_user_id(), true );
	}

	if ( null === WC()->cart ) {
		WC()->cart = new WC_Cart();

		WC()->cart->get_cart();
	}
}

I tested on fresh wordpress / storefront installation and the problem remains.