Some CDN rely on DNS for load balancing. They need information on end user network to properly resolve ( sending you to a server in the right geo for instance)

Some CDN rely on DNS for load balancing. They need information on end user network to properly resolve ( sending you to a server in the right geo for instance). If you use a third party resolver and it is not providing enough info, you may get poor performance. That's not some fancy tracking, in the end the CDN will get your IP and traffic. (Not saying it's impossible to use it for tracking purpose)