Looks like the threshold cannot be higher than the smallest value in the stacked series.
As a workaround, you can set the closedStacks property to true. This causes the bottom line of areas to follow the threshold line rather than the previous series. As a consequence, the areas don't look stacked, because they are overlaid on top of each other. So it may look better if fillOpacity is 1.
Here's how others solve stacks across the threshold. Both Excel and GS solve this the same way, by letting the second series be stacked on top of the first one even if the first starts in a negative position.