Every Webflow Collection comes with 3 date fields, including...
...but these fields do not work as you'd expect.
They are internal fields that are automatically updated for Webflow's own purposes.
Bloggers often confuse that.
Published Date in particular looks like it should store the date you first published your article- and it does. But it also updates every time you edit and re-publish that article, because to Webflow, it's considered "published," again.
I'd encourage you to avoid those fields completely.
Create your own Date Published field, and use that instead.
@vincent ran some tests here, and has a good explanation here.