I do not believe that Run ever represents a line break. To represent a line break, different kind of Inline is used - LineBreak.
Your text doesn't have any line breaks when viewed in FlowDocumentReader:
<FlowDocumentReader>
<FlowDocument>
<Paragraph xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation'>
<Run xml:lang='de-de' xml:space='preserve' />
<Run xml:lang='de-de' xml:space='preserve' /> das ist text davor
<Run FontFamily='Palatino Linotype'>
line2 dsf adsgf sd fds gs fd gsfd g sdfg df h g hdgf h fg hhgfdh gfh
</Run>
<Run xml:lang='de-de' xml:space='preserve' />
und das ist text danach
</Paragraph>
</FlowDocument>
</FlowDocumentReader>
Paragraph, on the other hand is a Block element which breaks to a new line. Perhaps Paragraph is causing your issue.
Why don't you use FlowDocumentReader to show FlowDocument text, instead of TextBlock? With FlowDocumentReader you would have full support for FlowDocument, including Paragraphs and all other TextElement-derived items.