Sometimes, the filtering control goes out of the screen when there is not enough space for it. There are two possible solutions for this problem:
1. Removing the distinct values listbox or the field filter controls.
2. Reducing the height of the distinct values listbox.
More information about these approaches can be found in the following troubleshooting article - https://docs.telerik.com/devtools/wpf/controls/radgridview/troubleshooting/filtering-control