Does Watch Series 9 have an upgraded heart sensor?

Hello bicycledave,

Yes, the Apple Watch series 9 features a third-generation optical heart sensor, while the earlier models had a second-generation optical heart sensor.

The "Technology" section in the following article confirms it: Apple Watch - Compare Models - Apple.

You are right: the third-generation optical heart sensor was introduced with Apple Watch Series 6. The Apple Watch SE still has the second-generation optical heart sensor. This is what got me confused.