This article is relevant for the port "New IP on every request" with the Unmetered Residential, USA, or Global datacenter service.
If your IP is not rotating this is due to a limitation with all known browsers such as FireFox, Chrome or Safari.
The reason your IP is not rotating is because your browser doesn't actually close the TCP connection after the initial request is completed.
Instead they keep the network socket open for future requests.
This means we are unable to detect when you are making a new request because they essentially reusing the same connection socket over multiple times.