Hi,
This solution is based on storing the value in a text field and the link to the lookup list is not “live”. This means the selected value will not be updated when you change or delete the original item in the lookup list.
When you choose to edit the list item with the SPJS-Loolup solution, your previous selections will be reevaluated, and any invalid options will be cleared – and the dropdown will fall back to “select a value”.
If you want to preserve invalid values when editing an existing list item, you can look at the cascading dropdown solution as this has an option to allow this.
Its not obvious, but if you use it with only one “level” you have a single lookup.
Hope this answered your question.
Alexander