amCharts has no area series: an area chart is a LineSeries
whose fills template has been made visible. That fill is the
only runtime signal there is, so it is what the adapter reads — a line
with no visible fill stays a line.
A stacked area draws two magnitudes at every sample: the band's own value, and the running total its top edge sits at. MAIDR announces both, which is why the stacked variants are trace types of their own rather than a line with a fill.
The same bands with valueYShow: "valueYTotalPercent", so each
one is a share of a common total.