I made my first Blogpost and I found a bug, that the code snipet was highlighted while I was editing the post or in preview mode. But when I open it for example on my mobile or in private mode or just in the same browser in "normal" view, the syntax hightlighting was gone.
This is how it looks in edit or preview mode:
And this is how it looks in "normal" mode:
I thought, this would be a bug, but at the end it turned out, that this is a feature.
I contacted the Plugin Author through the support forum and this is his answer:
I followed his instructions and now the sourcecode shines highlighted as expected!