Created attachment 26797 [details]
Test document

Two words separated by a line break character are glued together.

I tried to debug the issue and found a code in XWPFRun.toString() method:

if (o instanceof CTEmpty) {
   // Some inline text elements get returned not as
   //  themselves, but as CTEmpty, owing to some odd
   //  definitions around line 5642 of the XSDs
   String tagName = o.getDomNode().getNodeName();
   if ("w:tab".equals(tagName)) {
      text.append("\t");
   }
   if ("w:br".equals(tagName)) {
      text.append("\n");
   }
   <...>
}

The issue is that "o" is an instance of CTBrImpl, not CTEmpty. So this element is ignored.

Attached a test document.