Thank you for pointing out this issue. The Placement property values on the Popup being ignored is in fact a bug in RadMenu for WPF.
I have logged this issue. Please, vote for it in our PITS
I am afraid at this point there is no workaround to the problem. We will try to fix this issue in one of the nearest releases.
I have updated your Telerik points for your appreciated feedback.
the Telerik team
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