A stream can only have 1 id. If you’re getting a new id returned from the Get Streams endpoint then it’s because the broadcaster had established a new connection, which can happen completely automatically by most broadcasting software if there’s a momentary connection drop, or issue which caused the connection to be lost.
If you’re getting back API requests with the old ID still, it’s because all API endpoints use caching, and if you poll the excessively fast (faster than once a minute) it’s possible to hit one cache on a request and get the latest data such as a new stream ID after a connection drop, and then on the next request you may hit an old cache where it still has the previous stream ID because you were polling too fast.