Hi there,
Without the ability to examine your site directly, we are limited in how we can help.
I’ve checked on my test site and when I look at blog posts I do not see any lines at the bottom of blog posts. Screenshot here: https://d.pr/i/8wm3iY
I also checked static pages on my test site (which you mentioned were an issue on your site) and did not see it there either.
Since there is not a line present in the demo site or my test site, it sounds like this may be an issue related to a plugin you are using on your site. With this in mind I recommend disabling all plugins on your site as a test. Does that make it go away? If so reactivate your plugins one at a time (reloading your site in-between to check) and see if it returns. If so it is usually due to the last plugin you activated.
Hope that helps. Please let us know if you have any more questions.