Why rank tracking changed
A rank is a position in a list. That model fits Google's blue links and nothing else on a modern results page. When ChatGPT or Google's AI Mode answers a buyer question, there is no list to hold a position in: the engine writes a paragraph and names three to five brands inside it. You are either part of the answer or you are absent, and no classic tracker has a column for that.
The practical consequence: a brand can hold its Google positions perfectly steady while losing the AI half of its market, because the assistant answering the same question never mentions it. Tracking one without the other measures half the battlefield and reports it as the whole.