Cómo concatenar etiquetas similares en un XML

Aquí hay una forma de resolver el problema. No codifica ninguno de los nombres de nodos secundarios (TI , MO , AU , etc.) por lo que potencialmente podría usarlo en documentos similares. He puesto comentarios en el código, así que léalos y pregunte si no entiende lo que hace el código.

$txt = 'your XML string goes here';
$dom = new DOMDocument;
$dom->preserveWhiteSpace = false;
$dom->formatOutput = true;
$dom->loadXML($txt);

$xp = new DOMXPath($dom);

# find all the nodes types that appear under Record
$cnode_type = array();
foreach ($xp->query("/Results/Recordset/Record/*") as $c) {
    $cnode_type[] = $c->nodeName;
}
# cnode_type now contains the different child node types
$cnode_type = array_unique($cnode_type);

# get all the Record nodes
$recs = $xp->query("/Results/Recordset/Record");

# for every Record node...
foreach ($recs as $par) {
    # for each type of child node...
    foreach ($cnode_type as $c) {
        # run an XPath query to count the number of children of node type $c
        # if there are more than one, we need to remove the extras
        if ($xp->evaluate("count($c)", $par) > 1) {
            # go through all the $c nodes, saving the value in $node_vals
            # delete the node
            $node_vals = [];
            foreach ($xp->query($c, $par) as $n) {
                # only save the contents of nodes with a value
                if (isset($n->nodeValue) && strlen($n->nodeValue) > 0) {
                    $node_vals[] = $n->nodeValue;
                }
                $par->removeChild($n);
            }
            # create a new $c node and set the value to the list in $node_vals
            # add it to the parent node
            $new_node = $dom->createElement($c, implode("; ", $node_vals));
            $par->appendChild($new_node);
        }
    }
}
# print out the result
echo $dom->saveXML();

Salida del XML que ha publicado:

<?xml version="1.0"?>
<Results>
  <Recordset setCount="3">
    <Record setEntry="0">
      <TI>Test-1</TI>
      <MO>Mo-1</MO>
      <JF>OK</JF>
      <JT/>
      <AU>One; Two; three</AU>
    </Record>
    <Record setEntry="1">
      <TI>Test-2</TI>
      <MO>Mo-2</MO>
      <JF/>
      <JT/>
      <AU>One; Two; Three; Four; Five; Six; Seven</AU>
    </Record>
    <Record setEntry="2">
      <TI>Test31</TI>
      <MO>Mo-3</MO>
      <JF/>
      <JT/>
      <AU>One</AU>
    </Record>
  </Recordset>
</Results>