Hi Andreas
Thanks for reaching out on Lucidchart Community!
If you are not seeing the master page reflected in the other pages in your editor you may not have applied it to those pages. Master pages are applied to each page individually using the arrow menu on the page tab. See the gif below.
If however you do see the master page reflected in your other pages in your editor but not in your exports I would recommend trying it in a private or incognito window to stop all cookies and browser extensions. If the issue persists feel free to reach out to our support team with this form and we will take a closer look.
For anyone else looking for more information on Master Pages see this article: Master Pages