I have tested the described scenario where the RadDropDownTree is with width 100% and its parent DIV element is with style width: 30%. Then, when the browser width narrows, both the selected text or the Empty message get cut off with an ellipsis.
If that is not your scenario, please modify the attached project to replicate the issue you describe and attach it to an official support ticket. That would allow us to investigate locally your exact scenario and help you more efficiently.
Virtual Classroom, the free self-paced technical training that gets you up to speed with Telerik and Kendo UI products quickly just got a fresh new look + new and improved content including a brand new Blazor course! Check it out at https://learn.telerik.com/.