Thank you for writing.
This appears to be an issue in RadDropDownList. I have logged it in PITS and you can find the PITS record here: http://www.telerik.com/support/pits.aspx#/public/winforms/12874
. We will address this in our next release.
I am not able to provide you a suitable workaround due to the nature of the issue.
All the best,
the Telerik team