Could you attach the view in which you use the DropDownList, or at least post its container (a div element for example), along with any CSS you apply to your page. It would allow us to check how the DropDownList behaves in this scenario and whether any custom CSS might be having unwanted effect on its appearance. Also, since you point out the filter option specifically, could post a screenshot showing how the DropDownList looks like without filter set at your end?
Looking forward to your reply.
Telerik by Progress