“An unclosed (multi)polygon will not render, nor should it” - yes I know, hence me posting looking for help in debugging it 
- I suspected I’d missed a bit somewhere, but I couldn’t find it!
As regards ‘not tagging for the renderer’ - I know that… (although tbf, that’s not quite the same as taking a tagging scheme you read on OSM Wiki as gospel - there’s loads of out-of-date stuff on there, where people dreampt up some tags a dozen years ago but hardly anyone used or supported them)