An empty are before the row appears when you collapse its row details. This happens if the GroupRenderMode is set to Flat and the control is hosted in a RadExpander control. To recreate this, the RadGridView also should have its scrollbars displayed and a RowHeight set to a value bigger then 30 (ex: 35). Note that the row height may increase also if the height of the content in the row is bigger. 
Additionally, to reproduce this, the content of the row details should exceed the viewport.

To work this around, you can set the GroupRenderMode to the legacy Nested layout. Alternatively, you can use the native Expander control instead of RadExpander. Or you can ensure that the contents in the row is less then 35.