Hi Jens,
Thank you for contacting Syncfusion support.
We have checked your requirement “Month cell selection shape not applying for custom view” and which is the expected behavior. Selection shape can be customizable, which will work only in the default month cell view. When setting custom view, default view will be replaced with the custom view, so the default month view settings selection won’t apply. For custom view selection, you must need to handle by your own, which is the expected one.
Please let us know, if you have any concern.
Regards,
Subburaj Pandian V