The two DOCXs you posted use different Templates hence Styles. 
NEW INSTALL TEST1 
 
TEST 1 this work
 
Consequently we end up with different stylesheets and xhtml file break up in the EPUB
 
 
I convert up to a dozen DOCX to EPUB a week. The only thing I do to the EPUBs is to remove superfluous metadata from the OPF and rename the .XHTML files to match chapter numbers and or names.  I never touch the formatting.  If I see something wrong I correct the manuscript DOCX and redo the conversion.
BR