This is related to MAGNOLIA-2851 and MAGNOLIA-3007. In the latter PageIterator.initContentIterator() was changed to begin with:

Content activePage = MgnlContext.getAggregationState().getCurrentContent();

This means that if you use the tag while a non-page content node is active, it will come up blank. That struck me as very odd when I ran across it. It seems more appropriate to change it to this:

Content activePage = MgnlContext.getAggregationState().getCurrentContent();
try {
  while (!activePage.getItemType().equals(ItemType.CONTENT)) {
    activePage = activePage.getParent();
  }
} catch (Exception e) {
  // oh well
}

This will search up the tree until we find the page we're on, then iterate over its subpages. The try/catch is there because getParent() can throw JCR exceptions.