Hi Augustwind,
Thanks a lot for your code Snippet.
I created a sample using your code snippet but I am afraid that I was not able to reproduce the issue.
With this I have attached a sample for your reference.
Could you please try to modify the sample and send back to us, so that I can reproduce the issue. This will help me to analyze this issue further.
Thank you for your interest in Syncfusion products.
Best Regards,
Bhuvaneswari.
PDF_Merge.zip