Here’s an example of a transaction page showing notes field and reference field. Like you say, the reference is used as default text for the notes field.
But when you edit the notes field it also edits the reference as well as the grey text at the bottom of the transaction details page (as shown in images below). I’m not sure why anyone wouldn’t want these to be immutable records of the transaction that took place and so untouchable by edits in the notes field
Surely it would simply involve mapping the reference field in the UI to the “description” field in the underlying data structure, which doesn’t seem to get modified when the notes are modified