Yes, your sample works correctly, issue is not reproduced. I was updated Essential Studio, but it did not change anything. I think I found the source of the problem. I am using AutoComplete control in TabItem of TabControl and first item is automatically selected when I select appropriate TabItem. But if I take the AutoComplete out of the TabControl then everything works fine. How the TabControl can affect the behavior of the AutoComplete?
I was able to reproduce the problem on your example. All works correctly only if the AutoComplete located is in the first tab. See attachment:
Ok, Thank you!