yes the stacking order is always the first sorted series on the bottom and the rest on top. You cannot change that in Kibana.
A small side-node from data visualization aspects: You're currently using a stacked line chart, which we usually would not recommend doing (and thus also not allow it in our newer visualization editor Lens). I'd recommend if you want to use line visualizations to disable the stacking under the "Metrics & axes" tab, switch the "Mode" to "normal" instead of "stacked":
Since the area under the line is not filled, using "stacked" mode can be really confusing for the viewers of your visualization, why we'd in general recommend using area charts if you want to have it stacked, or using normal mode if you want to use a line chart.