I don't know the answer to this, but I have confirmed the same behavior. I initially thought it was limited to 800x480 devices (HD2), but have since confirmed on all of the WM6.5.3 emulators. Here's a link to my ongoing investigation of the issue:
http://nomadelectronics.blogspot.com/2010/05/problem-with-800x480-resolution-on-htc.html