We cannot really know what happend before or why all the collection items got the same published_on date. My guess is, that you imported the collection items from a different project and hit publish. Then the published_on reflects exact moment when its actually published on the page (in webflow).
Another guess is, that all of the collection items were drafts before and not published to the specific / new domain.
To solve this I think its the easiest way is to use the changed_on date in general. But i guess you dont want that, because you don’t want to have updated articles shown as the newest article…
I guess the only way is you export the Collection items, modify the CSV and import it again. If the imported items are still listed as “not published” then you will be at the same problem i guess.
Or like mentioned here, to create a custom field as “Original Date”: