Ok I see, this is expected. And IMHO it’s a good thing, here’s why:
The published date is updated each time you’re editing an item. You can read it as “the published date of the actual version of the CMS item”. Published date as you expect it to be would be the “original published date” as often seen on some sites and CMS. I think it’s OK because htis way you can’t be fooled about how old really is an item.
Anyway, those 3 date fields are not necessarily meant to be shown in your pages, they’re primarily here to help you keep track of your posts and to sort your lists. You have all freedom to craft as many date field as you want (I call them Editorial date fields) and manage them as you want.
On all my Articles and Posts collection, I have an “Originally published on” date field, and I also use the default “Updated on”. I set the first manually and the latter remains automatic.