Detected HR too low while running

The sudden HR changes make me think of improper fit of the watch or maybe lots of sweat beneath it.

C.

It's fitted properly, as far as I can tell.

its important to reboot the watch once a week to improve performance, and wear it few cms over the wrist bone tight

Whats your cadence? Could be cadence lock.

Is the sensor clean, unscratched?  Do you have a lot of hair, tattoos or similar on the wrist?  Is it tight and high enough?

I don't think it's any of those, but I do want to get a stretchy nylon strap to see if that could help. This is my 2nd 255 and the previous one was fine.

Same situation here.

Sensor clean and watch tight over the wrist.

Same question - whats your cadence for the same time?

I don't think it's cadence lock.

Same problem. Drop from 150 to 135-140, next 10 km at this level, and at the end of 2 hours run, show 165-170. Its not first time, appears after software update in october ️

Here is my cadence vs pace vs HR at the same time: 

I do not know if it is related or not, but this started to happen in October.