A short follow up to this.
I suddenly had a thought, and went back and checked the components involved. Sure enough, my thought was correct - I hadn't forgotten to set the default value of the Select component.
The reason it wasn't set previously is because all the component in the table's expanded row are inside a Form. So the Form receives the data from the currentRow and serves the data to the components by name (Form data key).
But somehow this wasn't passing the null through correctly in the Select component.
I'm not sure how setting Default value works for a form component which is already tied to a value received by the Form - but it did help get the null handled correctly.
For now I'll leave it at that.