There's an issue with the RadMultiColumnComboBox in that the inner text field is left aligned so if the user doesn't click exactly in the text box, then they can't type anything.
In the attached image, I set the border of the inner textbox to red just for a visual aid. If the user clicks anywhere inside the red border, then the field gets focus and they can type and everything works as expected. However, if they're not focused on that field and they try to click to the right of the red border, it does nothing and to the user, it feels buggy.
I tried setting the horizontal alignment of the text box to stretch, but that had no effect. Is there a fix for this?