Created attachment 42530 [details]
Copy of a chapter in the Writer Guide
I tried File > Send > Create Master Document on a document with multiple level 1 headings. No matter what I chose as "Template" (Outline Level 1 or 2, or the specific heading style used in the document), the original file was duplicated as an .odm file. No subdocuments were created.
Sorry, this apparently depends on using the LibreOffice dialogs for Load/Save. The splitting works properly with that set.