If you're using line-connected graphs (setLineStyle not lsNone), the adaptive sampling will make sure all spurious peaks are displayed properly. In the case of scatter points only, points in the middle of a vertical range are thinned out which does lead to a visual change. However, even there spurious outer points should be preserved.
It would be great if you could provide me with a specific configuration (e.g. an example code which generates the data and plot setup) where you observe the behavior, with screenshots -- because leaving out outlier data points is exactly what the adaptive sampling algorithm explicitly avoids, so this is a bit strange and possibly a bug.
You can read more about adaptive sampling (with screenshots how it's supposed to behave with outliers) in the QCPGraph::setAdaptiveSampling documentation. To disable the adaptive sampling, call