XWPFDocument.insertTable does not seem to work at all.  Created small example word doc that looks like:

0: Paragraph 1
1: Table 1
2: Paragraph 2
3: Paragraph 3
4: Table 2
5: Paragraph 4

The indexes are what's in the bodyElements array.  Verified its being read as expected. Then running this example on it:

try (XWPFDocument doc = new XWPFDocument (getClass ().getResourceAsStream ("/InsertTable.docx")))
{
	// This is just here to verify the indexes into the bodyElements list are as expected
	int index = 0;
	for (IBodyElement elem : doc.getBodyElements ())
	{
		System.out.println (index + ": " + elem);
		index++;
	}
			
	// Copy one of the existing tables and insert it at specific point
	XWPFTable src = (XWPFTable) doc.getBodyElements ().get (1);
	XWPFTable dest = new XWPFTable ((CTTbl) src.getCTTbl ().copy (), doc);
	doc.insertTable (3, dest);

	File outFile = File.createTempFile ("TestInsertTable", ".docx");
	outFile.delete ();
			
	try (FileOutputStream out = new FileOutputStream (outFile))
	{
		doc.write (out);
	}
}

The copied table doesn't come out in the word doc at all.  Moreover if you look at what insertTable is doing, it adds the new table into the bodyElements list, but that's just an ArrayList so doing so doesn't trigger any special processing or updates of the underlying XML.  So then it is searching for where the newly added table XML (CTTbl) was added into the document XML, and fails to find it because nothing added it.  So then it defaults to adding the table at the end of the list of tables, but again this is just an ArrayList so isn't triggering any special processing or updating of the underlying XML.

If I use createTable instead of copying an existing table, the table does get added, but at the end of the doc instead of at the desired index into the bodyElements list.  This is because createTable() has already called ctDocument.getBody().addNewTbl() and added at the end of the XML before we call insertTable to say where we actually want it.