Interesting. This could be several things. :-)
A.
Somewhere in your code, you are using this rotation value and sticking it in somewhere. But the place you are using it could need a different format than what you put in (e.g. You may be sticking your values in the wrong place).
B.
This could be a bug in WPF. There are small bugs like that, littered throughout C# (e.g. when using foreach loop, you cannot modify the values, because that will cause the program to crash). So you might want to Google this, and see if this is a well-known problem.
C.
Maybe you are using the wrong technique to rotate (no offense intended). Check on MSDN (
http://www.msdn.microsoft.com/),
and look in the Library, in the System.Windows section (
http://msdn.microsoft.com/en-us/library/gg145013.aspx), and search through the classes to see if there is a more appropriate way to do 3D rotation.
Hope this helps.
:-)