But that's WordPress doing that, or am I mistaken? I mean, on all sites where I use
/%postname%.html permalinks for posts, pages will not get that .html extension... Even when I put that in the slug name manually, it will get converted to
/%pagename%-html (notice the hyphen instead of the dot) immediately. Whether or not I am using the XML Sitemap plugin, does not seem to make a difference.
But I must say all my sites are on WP in Multi-site mode so it might be that WP in standard mode behaves differently in this respect... Or are you running some plugin that allows for more complex Permalink parameters?