Support Forum

After installation of the plugin, from the admin screen where you "use" Core Tweaks, there is an option at the bottom (off by default) to install a couple of other plugins  - Page Link and Page Order.

It sounds like one of both of those were installed, so you can either uninstall them from the same screen, or g oto their options in the admin – My Page Order is under "Pages" and Page Links is under Settings, where the excluded pages are likely unchecked.

However, Core Tweaks should have no effect on any "featured images" appearance -

If it did then I'd ask whether you tried any of the "Advanced Options" near the bottom of the screen, where you change file permissions and change the H1 / H2 tag?  I beleive that still wouldn't affect the featured image, but it's the only remote possibility…