I have a created a custom post type called Campaigns. Its slug is /campaigns, and post URLs are /campaigns/my-post-url.
I have a page called Campaigns where I want to display the list of posts. The page slug is /campaigns. I've used this same configuration previously without any issues.
I can create my layout of the post list on the /campaigns page. I'm using Oxygen Builder, though I don't think that matters. However, when I view the page in the browser, I get the content of one of the posts.