To reproduce: Add a RadMultiColumnCombobox to the form, set the DropDownStyle property to DropDownList and clear the text. You will notice that the height of the control is not bigger than 5 px. Workaround: set MinimumSize Me.RadMultiColumnComboBox1.MinimumSize = New Size(200, 23)
The issue can be reproduced in R3 2018 SP1 by simply setting the RadMultiColumnComboBox.DropDownStyle property to RadDropDownStyle.DropDownList and setting the Text to "".