I managed to make a hotfix, by replacing
if ( isset( $_POST['add-to-cart'] ) ) {//phpcs:ignore WordPress.Security.NonceVerification
$cart_item_data['ywgc_product_id'] = absint( $_POST['add-to-cart'] );//phpcs:ignore WordPress.Security.NonceVerification
} elseif ( isset( $_REQUEST['ywgc_product_id'] ) ) {//phpcs:ignore WordPress.Security.NonceVerification
$cart_item_data['ywgc_product_id'] = sanitize_text_field( wp_unslash( $_POST['ywgc-template-design'] ) );//phpcs:ignore WordPress.Security.NonceVerification
}
with
if ( isset( $_POST['add-to-cart'] ) ) {
$cart_item_data['ywgc_product_id'] = absint( $_POST['add-to-cart'] );
} elseif ( isset( $_REQUEST['ywgc_product_id'] ) ) {
$cart_item_data['ywgc_product_id'] = absint( $_REQUEST['ywgc_product_id'] );
} elseif ( isset( $_REQUEST['product_id'] ) ) {
$cart_item_data['ywgc_product_id'] = absint( $_REQUEST['product_id'] );
}
in class-yith-cart-checkout.php