Hello,
When you set mergeType as 1, the sections of the merged DOCX are removed. Only the first section is kept (you can read this behaviour on the API page of the method). If you set mergeType as 0, then all sections are preserved.
We recommend you to use the enforceSectionPageBreak available in the method. Or use DOCXPath to add new breaks in the merged DOCX.
Regards.