Actually, this would be the expected behaviour since UI Virtualization is turned on by default. It enables RadGridView to reuse the existing containers over and over again for different data items, instead of creating new ones.
Generally, it is not recommended to use GridViewToggleRowDetailsColumn if you have set RowDetailsVisibilityMode to Visible (if this is your case). An option for you would be to turn-off the virtualization. Although this is also not recommended, if you have small amount of data it may work for you.
the Telerik team