When exporting PDF documents containing images different than Jpeg and Jpeg2000 the PdfProcessing is using by default the ImageSharp library in order to convert these images to Jpeg.
It seems there is an issue in the older version of the ImageSharp library: Saving a PNG as Jpeg only processes a part of the image on .NET 6.
Workaround: This issue seems to be fixed in the current version (2.0.0) of the ImageSharp library.