Because of the built-in line skip, LaTeX can't handle a section header as the first element in a list item. (To be precise, it can't handle it if the list immediately follows a section header, but the instance is rare enough that we can afford to be a bit more general). This puts a non-breaking space before the header to solve this problem. We won't see this space, since the header skips a line before printing anyway. The output is ugly in LaTeX and this structure seems like it should probably be avoided. But it is valid HTML and native pandoc, so we should have some sort of typesettable representation in LaTeX.