There are multiple highlighted (background is blue) or selected (font is blue) rows in dropdown if it contains rows with rowId = null or rowId = “”. This happens if the actual selected value in the combobox is null. There is only one selected and one highlighted row if a valid row is se4lected in the combo box.
This is not a problem in the client or the server but rather a modelling problem. There should be a hint in the console pointing to the modeling problem.