HR mystery, possibly tech related

yirara

I'm trying to solve a HR mystery. When I run regularly I observe some strange things. It doesn't happen with every run, but when it does it's always after around 3km: suddenly the running becomes super easy. When I then look at my phone connected to an optical upper arm sensor my HR is about 15-25bpm lower than it was a few minutes earlier. When I'm not quite so fit HR comes up after a while again but running remains easy. Odd. As I get fitter the HR remains down there until the end of the run. I've seen this with 2 different optical sensors from 2 different producers, Scosche and Polar. I once had a lab test in a hospital, and there this odd thing happened as well for a few minutes. But: neither two different Garmin watches, nor my old Fitbit are able detect this! For both my HR remains high. Any idea?
(note, I don't have POTs. Any exercise is nearly impossible for me before my HR get properly up. Inborn muscle disease without diagnosis)

Here is a run from today:
Garmin Fenix 7s vs. Polar OH1