Why is Last Published On date blank in all my public answers?

Summary:

We have over 1600 knowledge articles in our customer portal visible to customers but none of them have a Last Published On date even though they've been published several times.

Content (required):

How do I get the system to automatically set the Last Published On date? Do I have to create a business rule to do this or is there a way for the system to do it automatically?

Version (include the version you are using, if applicable):

21C

Code Snippet (add any code snippets that support your topic, if applicable):