After further investigation we found that the issue is a limitation from ScrollViewer inside ScrollViewer scenario which is why the logged bug report is declined. Here is more in depth description of the issue and a workaround for it:
The issue is caused by the fact that the RadAutoCompleteBox's ControlTemplate has a ScrollViewer inside it which makes the described scenario ScrollViewer inside ScrollViewer. In this case the mouse wheel is handled in the inner ScrollViewer and cannot be passed to the outer one. The same behavior can be observed in the default ListBox control which also has its own ScrollViewer.
In order to workaround this limitation you can edit the ControlTemplate with x:Key="RadAutoCompleteBoxTemplate" and replace the ScrollViewer with x:Name="PART_ScrollViewer" with another control for example a Grid. A sample project of the workaround can be found attached to the feedback item. The project uses Implicit Styles, please make sure to refer the No.XAML assemblies.