Hello,
The issue isn't with phpdocx 10 but with the latest revisions of MS Word 2016, MS Word 2019, MS Word 365 and MS Word Online that have added ID limits when working with lists. Please check the information available on https://www.phpdocx.com/news/post/important-changes-for-lists-in-the-last-ms-word-update/221
All phpdocx 10 licenses were updated applying the needed changes days ago. We have tested your case (merging a blank DOCX and a DOCX with bullets) and we get a correct output with all DOCX readers. If after downloading and using the latest version of phpdocx 10 you still have the same issues, please send to contact[at]phpdocx.com the DOCX documents you are merging, the output you get and the program you are using to open the DOCX and we'll check them.
Regards.