There was an optimization in the sitemap handler that didn't pan out as expected, so I've removed it. Specifically, if you didn't allow the "view node" permission for guests globally but allowed it for forums, it would just skip building threads.
This permission actually acts inconsistently from how it works when passing permissions down the node tree (if you can't view a parent node, you can never view the child node). This certainly isn't something that can change now due to backwards compatibility, but it could possibly be changed in a future (major) release.