Meta-Tags description and keywords in article node?

I'm using the dev-version (2012-Aug-22) with all sub-modules enabled. I'm getting both tags, if I'm viewing existing page nodes. Both tags are missing, if I'm viewing existing article nodes.

In the node type article I've enabled:

• Generate meta tag content when the meta tag content is empty (default)
• Generate meta tags content from the node teaser (default)

In the module settings I've enabled both taxonomies, which I'm using in the article node type. No other taxonomies are available.

Does anybody know why only these tags are missing? The other meta tags like dublin core, geo position and open graph are generated in both node types.