Powerpoint deletes relation of CustomXmlPart to Slide

Hello.

With the OpenXml SDK 2.5 I added a CustomXmPart to a SlidePart. I added an CustomXmlPropertiesPart and finally added an additional relationship from CustomXmlPart to the PresentationPart. On the first view the Powerpoint.pptx-File looks OK, but after opening and saving the file with Powerpoint the relation from the CustomXmlPart to the SlidePart has gone from the file and is not found programmatically either.

I compared the file to files we created with the Office.Core Library and seemed to have the same structure now.

Did I forgot a relationship or anything else?

Thanks for help.