When an action is completed, the message "Do you want to add a follow up action?" appears. Select "Yes."
The follow up action shows the note of previous actions. To see this, select the Notes tab on the Action, and the previous notes are located in the NotePad section.

If Notepad Description and Notes is included as optional columns for the Action tab, the first notepad added to the action will display.  Since the Follow Up action will have the Notepad from the original Action, it is the first Notepad on the Action and will be what is displayed on the Action tab.  If additional Notepads are added to the follow up action, they must be viewed by opening the Action record.  This is working as designed.