The heart rate zones in the Polar Flow app are misleading. Here’s a recent run.
The problem with the Polar app is that the zones are based on assumptions which may not apply for you. More specifically your fuel is a mixture between fat and carbohydrates and the hard edges these applications show don’t reflect a mixture.
I had my own VO2max tested in a lab. I know what my fuel mixture is at a particular heart rate. My Polar app shows me in fat burning below 111 bpm and “fit” above that point. My MAF range is 112-122. My VO2Max test showed my fat/carb burning at 90% / 10% at a heart rate of 124.
But I am fat adapted keto for two years so I am primarily a fat burner. This is why MAF works well for me. My 100% fat burning heart rate is 117 which center of MAF. As long as I am in the MAF zone I’m burning nearly all fat – even though the Polar Flow program says otherwise.