For a custom ScrollViewer with that modifies the scroll position in response to a ScrollChanged event (such as to create an AcceleratedScrollViewer that increases scroll speed over time), the modified scroll position is immediately overwritten by OnMouseWheelExternal() for as long as the scrolling continues. The modified scroll is only applied on the last frame when the scrolling stops, as there is no longer an overwriting OnMouseWheelExternal() call. This seems to be a timing issue stemming from a recent change to ScrollViewer.

Here is a sample of the two functions in question that demonstrates the issue:
FastScrollViewer::FastScrollViewer()
{
this->ScrollChanged() += [this](BaseComponent / sender */, const ScrollChangedEventArgs& args) {
const float delta = abs(args.verticalChange);

    if (delta > ACCELERATED_SCROLL_INCREMENT_BASE)
    { // filter out returning accelerated scroll increments, as in: calling ScrollToVerticalOffset will call ScrollChanged again
        return;
    }

    ScrollToVerticalOffset(GetVerticalOffset() + (float)sgn(args.verticalChange) * delta * 1000);
};

}

void FastScrollViewer::OnMouseWheelExternal(float wheelRotation, Noesis::Orientation orientation)
{
if (orientation == Noesis::Orientation::Orientation_Vertical)
{
ScrollToVerticalOffset(GetVerticalOffset() - wheelRotation / 3.0f);
}
}