The grouping with collapse = TRUE is working when I have only one listviewwebpart but when I add a second xsltlistviewwebpart on the page the grouping stops working. I see the groups collapsed in the Design view of SharePoint designer but not in the browser. I tried Ctrl+F5 to clear the cache and still not working. I think this collapse property doesn't work anyway because it will render the previous state on the next the page refresh.
Is there a solution or this one of SharePoint's bugs?