As per the screenshot below, I have targeted the third item of the Nestable accordion using :nth-child . In the builder the style is applied to the second item. But in the frontend it is applied to the third item just fine.
thank you for your report. This happens because in the builder, we have an additional HTML element .brx-nestable-children-placeholder (it’s important one, without it it would not work in the builder), so the HTML markup is a bit different in the builder than on the frontend.