heart rate including afib will report to an Iphone 12?

FitBit Sense 2 works with iPhone that has an operating system of OS 14. I have an iPhone 7 with an OS 15 and it works just fine. Hope this helps.

Why did the price change back to full price yesterday from $79.95 when I attempted to purchase two of these items?

It was a "DOORBUSTER" for black Friday/cyber Monday The price was SIGNIFICANTLY below even wholesale price. Prices of retail products fluctuate EVERY DAY, even SEVERAL TIMES A DAY. This could either be the retailer OR it could be Amazon OR could be the product company. In the case of the super sale on the FitBit Sense 2, the sale was ONE PER CUSTOMER and ONE PER ADDRESS. I know, I tried to do the same. (I did get one). The idea was cooperative with Amazon the retailer as part of the black Friday/cyber Monday sale AND Google (FitBit) helped absorb most of the price. They were trying to increase their customer base and the Sense2 was a promotion in order to try to hook users on to a subscription model of profit raising. They just don't want a 200 dollar sale, they want to get you to pay 80-120 a year. Like ALL products, whether it be a piece of software, or a peloton, there are upfront costs AND back end costs. Even though you and I were trying to buy gifts, there were other people doing what is called "retail arbitrage"> There were independent retailers that thought they could buy a 65 inch TV, a 50 inch TV, a Fitbit, all below wholesale price, and then re-sell it

Is it water resistant?

Depends on what kind of water resistance. I shower with it and no problems but the screen keeps waking up and it seems like water triggers it to push some buttons on the screen... I think this can survive a swimming pool too

Which features are premium only?

Daily readiness score, a breakdown of stress management score, health metrics trends for past 90 days, wellness report, advanced skin temp. Details, over 200 workouts and mindfulness sessions, a breakdown of sleep score and, blood glucose trend which you need a comparable glucose meter and Bluetooth .