In the theme editor, when you click on some menu (Footer, Purchase Options, Homepage, Newsletter Popup), a popup is shown out, but it is not correct with the menu you have clicked.
Our theme is based on Cornerstone theme (BigCommerce official theme), the Cornerstone developers had added the new attribute
to the schema.json file, it's reason cause this issue. I have reported this issue to the Cornerstone developers, just wait for them, it's may a bug or the new feature that they are building.
You can report this issue to BigCommerce that the "Payment Buttons" section doesn't show in theme editor, let them know the Cornerstone theme has same issue. They may check it and fix it soon.
If you want to configure the Newsletter Popup, you can switch to the old theme editor and continue your work