Consider the following router configuration:

scope "/" do
  pipe_through :browser

  get "/swagger-ui", OpenApiSpex.Plug.SwaggerUI, path: "/api/openapi"
end

scope "/api" do
  pipe_through :api

  get "/openapi", OpenApiSpex.Plug.RenderSpec, []
end

This works perfectly until a non-empty value is provided for the path parameter in the Phoenix.Endpoint configuration url parameter. Once a non-empty path is provided, the SwaggerUI plug substitutes the incorrect path in the generated HTML.