I think this is a new issue that has come up with ASP.NET 4.0. I can reproduce it on my computer with IIS 7.5. If the application pool is at 2.0, then the URL comes thru as having default.aspx in it (even when default.aspx is not in the URL).
But if I change the application pool to ASP.NET 4.0, default.aspx does not come thru in the URL (reported to ASP.NET).
At least this is true for "Request.RawUrl", but "Request.Path" does correctly show default.aspx even in ASP.NET 4.0.
For the front page check, BE is looking at RawUrl and not seeing default.aspx when under ASP.NET 4.0.
I would suggest changing the application pool from 4.0 to 2.0.