customizeWordContent uses w:body as default parent, so only the contents that have w:body as parent will be changed.
On the API method doc page (https://www.phpdocx.com/api-documentation/docxcustomizer/customize-docx-Word-documents-PHP), you can find explained the following option:
parent string Main document body as default, allows to set any parent or a specific one. w:body (default), '/' (any parent) or any other specific parent (/w:tbl/, /w:tc/, /w:r/...).
If you want to apply customizeWordContent to all contents, not only the ones that have the main w:body as parent, for example when you embed a list in a table, you can set parent to '/' (any parent), or a specific parent (for example '/w:tc/' for cells). If you don't know the specific parent, we recommend you to use the global '/' parent.