This happens in 6.00 RC1, not sure if 5.00.1 had this issue.
I haven't seen such behaviour in any of more than 50 editors I tried, so I consider this a bug.
When you select code with Shift-PageDown/PageUp and the remaining unselected code is less than enough for page scroll action to happen - it won't be selected.
For example, place cursor at the first line of any long enough file and then try to hold Shift-PagDown until the editor area is scrolled to the end of the file.You will see that some code at the end remained unselected and it cannot be selected using Shift-PagDown.
I can't see how this can be considered a bug by any definition, but I will look at it when I can...