Bringing this thread back up, because I've noticed something similar. The last two years, I keep seeing HR numbers a lot lower than I'd have expected for the speeds I'm going. A bit of context - before the Pandemic I was doing about 1000-1500 miles a year, all outdoors. In mid-2020, what with WFH, I started riding a lot more. 2021 and 2022 were both over 6000 miles, 2023 - 2025, between 4500 and 5500/year. I used intervals.icu to plot Power vs HR for every year since 2021. CLEARLY there's a big change between pre-2025 and post 2025.
Doing FTP tests (used to do Ramp, but more recently The Grade on Zwift), I get the pretty much the same number.
This is all indoor rides, on a Smart trainer, but I feel the same thing outdoors - at the same RPE,and the same speed, my HR is a good bit lower. I don't feel any change in fitness/ability. That is, mostly I'm doing the same times on the same routes, feeling like I'm working about as hard, but instead of 150 bpm, I'm seeing 135. Of course this plays merry hell with HR zones, which I need to go back and refigure.
Terry and I are about the same age, BTW.