On an intranet I have been working on recently I came across a bug where the display of an External Data field in the 1st and 2nd level grouping is not shown when grouping by the BCS based item(s).
To provide a short term fix I created the following JavaScript to run on the page to solve the problem.
1 <script type="text/javascript">
2
3 _spBodyOnLoadFunctionNames.push("updateGroupNameValues_Level1");
4
5 function updateGroupNameValues_Level1()
6 {
7 $('TBODY[id*=titl]').each(function()
8 {
9 var currentGroupString = unescapeProperly(($(this).attr("groupstring")));
10 currentGroupString = currentGroupString.replace(';', '');
11 var groupStringArray = currentGroupString.split("#");
12 var groupLevel1 = groupStringArray[1];
13 var groupLevel2 = groupStringArray[2];
14
15 var currentSpan = $(this).find("span").text();
16 var newSpan;
17 if (groupLevel2 == "")
18 {
19 newSpan = groupLevel1 + " " + currentSpan;
20 $(this).find("span").text(newSpan);
21
22 }
23 else
24 {
25 newSpan = groupLevel2 + " " + currentSpan;
26 $(this).find("span").text(newSpan);
27 }
28
29 //alert("groupLevel1: " + groupLevel1 + " | groupLevel2: " + groupLevel2);
30 //alert(($(this).children("span").html()));
31 });
32 }
33
34 </script>
35
Essentially the values do exist in the DOM / HTML, but they are not displayed in the tags presented by the grouping.
NB: Please be aware that this solution relies upon JQuery.
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SharePoint (and Project Server) Shenanigans 
Thank you. Your solution seems to me could be better supported then one suggested in http://social.technet.microsoft.com/Forums/en-US/sharepoint2010setup/thread/a73896a4-43b4-474e-8fb1-c8a7ccf7056f/
this fixes the duplication of text when mixing BDC and non-BDC grouping:
function updateGroupNameValues_Level1()
{
$(‘TBODY[id*=titl]’).each(function()
{
var currentGroupString = unescapeProperly(($(this).attr(“groupstring”)));
currentGroupString = currentGroupString.replace(‘;’, ”);
var groupStringArray = currentGroupString.split(“#”);
var groupLevel1 = groupStringArray[1].replace(‘;’, ”);
var groupLevel2 = groupStringArray[2].replace(‘;’, ”);
var currentSpan = $(this).find(“span”).text();
var newSpan;
if (groupLevel2 == “”)
{
newSpan = groupLevel1 + ” ” + currentSpan;
$(this).find(“span”).text(newSpan);
var tdtext = $(this).find(“td”).text().replace(groupLevel1 + groupLevel1,groupLevel1);
$(this).find(“td”).text(tdtext);
}
else
{
newSpan = groupLevel2 + ” ” + currentSpan;
$(this).find(“span”).text(newSpan);
var tdtext = $(this).find(“td”).text().replace(groupLevel2 + groupLevel2,groupLevel2);
$(this).find(“td”).text(tdtext);
}
//alert(“groupLevel1: ” + groupLevel1 + ” | groupLevel2: ” + groupLevel2);
//alert(($(this).children(“span”).html()));
});
}