during usability studies we observed that when a lookup field is pre-filled, it's not immediately clear to editors that the initial value is something that has been pre-selected from a defined list of values. the issue seems to be related by the visual appearance of text in a lookup, which looks exactly like text in a text input component.
when editors try to edit this field, the experience is similar to editing text in a text field, but as soon as you remove one character from the text in the lookup, that entry becomes immediately invalid because it has been not selected from the menu.