I'm not quite positive that this is a bug, but I’m pretty sure it is.
When Template.render_context() is called, it sets context['self']. So, if one renders another template like this:
After the call to render_context(), context['self'] no longer contains the correct namespace. This (at least) causes problems for any <%block>s in the calling template.
I've attached a short test script which demonstrates the problem.