QSortFilterProxyModel: incremental insert is not stable

My application produces contiguous datapoints. I want to display them grouped by Day (TODAY, TODAY-1) in a QML TableView and within the days I let the user choose (by clicking on the column header).
Therefore I have chained two QSortFilterProxyModels. The outer one (which fills the view) has its sort role set to the day role and its sourceModel is the inner model. The inner model's sort role is set based on user input.
If the user happens to choose the datetime role as sort role and descendend sort order all new data points are added at the end of the day group. But they should rather be added at the front of the day group.
If the user triggers a new sort e.g. by changing the sort order. Everything is fine again, till new data points arrive.