Closing because of duplication with the following => https://feedback.telerik.com/wpf/1352386-chartview-stacked-areas-are-drawn-incorrectly-for-mixed-values
Scenario will be possible to achieve with setting chartview:ChartViewExtensions.StackNegativeValuesSeparately="False" on the RadCartesianChart in 2019 R3 SP and internal builds from 30 Sep 2019.
Also worth noting the issue is present in Stack100 AreaSeries as well: