To reproduce:

Create a RadPropertyGrid and set its SelectedObject. Set the theme to VisualStudio2012Light, change a property of the selected object and select another property, you will see that the previous property is selected. Now scroll until the previous property is no longer visible, now scroll back until it is visible, you will see that the selection is gone. By design this selection should not exist at all.

Workaround:

Open the theme with VisualStyleBuilder => RadPropertyGrid => PropertyGridElement => PropertyGridTableElement => PropertyGridItemElement and set the FillTransparent repository to the PropertyGridItemElement.IsModified state.