Both stackedarea and stackedbar charts ignore order of y axes and because of that appearance of the chart can't be changed. And that leads to charts that could be found harder to understand or to read data from.
Here's a live example: https://www.mediawiki.org/wiki/Google_Code-in/2017#Weekly_summary_of_tasks It was tried to move y1 (green one) on the top of the chart, but without any effect. Try to edit order of y axes on this chart. It'll be unchanged afterall.
We need to examine why such a bug occurs and fix it.