To improve sitemap generation performance, xmlsitemap_get_path_alias() skips calling url(). This has a side effect of skipping any hook_url_outbound_alter() implementations, notably in modules like subpathauto.
The attached patch fixes this, allowing for URLs like node/1234/photos to be properly exported as clean URLs in site maps. We currently have a custom module creating links for these paths, but I expect a similar situation could be recreated by installing subpathauto and adding a custom link through the UI for node/1/edit.