An element that's floated exists
outside the normal document flow. Clearly, when it comes to paging, ADE only considers the primary flow, and any elements outside that are restricted to the page on which they first appear.
So I suppose it's not really a bug as such, more a side-effect of css2's lack of any sort of decent page model. Other renderers (such as calibre's viewer) handle this in a more friendly fashion, however, so it's worthy of a note.