Hi Theo,
Thank you for your interest in Syncfusion Products.
It is not an error and it is intended to use multiple selection of items. You can preselect an item by using integer array.
For example:
int[] index= new int [1];
index[0] = 1;
listField.SelectedIndex = index;
Please refer to the below link for more details
Sample586529092.zipPlease let me know if you have any questions.
Thanks
Pradeep