Leading/trailing whitespace is preserved only if leading or trailing character is a whitespace but fails to preserve if that chararter is a tab (\t). Tab characters get stripped in the generated Word document.

Here's a patch to XWPFRun.java in org.apache.poi.xwpf.usermodel package from poi-ooxml-4.1.1.jar.

*** XWPFRun.java
--- XWPFRun.java.orig
***************
*** 146,152 ****
       */
      static void preserveSpaces(XmlString xs) {
          String text = xs.getStringValue();
!         if (text != null && text.length() >= 1 && (Character.isWhitespace(text.charAt(0)) 
! 											|| Character.isWhitespace(text.charAt(text.length()-1))) {
              XmlCursor c = xs.newCursor();
              c.toNextToken();
--- 146,152 ----
       */
      static void preserveSpaces(XmlString xs) {
          String text = xs.getStringValue();
!         if (text != null && (text.startsWith(" ") || text.endsWith(" "))) {
              XmlCursor c = xs.newCursor();
              c.toNextToken();