Hi Niels Cup,
Greetings from Syncfusion. We have validated your screenshot, we would like to let you know that currently we don’t have a Border support for SfChart control, so we suspect that the shadow effect on your screenshot is from Frame which might be the parent of your chart control in your application. So, can you check whether HasShadow property of Frame has been set to true in your application?
Since we are not able to reproduce this issue at our end, can you please provide us your code snippet or an issue reproducing sample so that we can investigate further and provide you a better solution at the earliest.
Thanks,
Michael