I haven't really thought about sectioning elements (<section>, <aside>, <nav>, and <article>) and the cool behaviour of a tag like <h1> when these tags are nested.
Validation tools will also tell you if you have an H1 in a <section>, but no H1 outside of the section, because the <section> actually creates it's own scope, thus your page has no H1.
Looks like that kind of validation was premature. They mention in this article that the “creates its own scope” part was never actually implemented for accessibility, this change brings the display rules back inline with reality.