My Cardio fitness levels are not updating anymore on iPhone ?

Which Apple Watch Series do you have? Is it still paired with your iPhone?

You do need an Apple Watch Series 3 or later paired with your iPhone to get Cardio fitness level readings. The Apple Watch uses various sensors to estimate your VO2 max, which is a measure of your maximum oxygen intake during exercise. This data is then transferred to your iPhone's Health app to display your Cardio fitness level.

While the iPhone app itself cannot directly measure Cardio fitness, it relies on the data collected by your Apple Watch.