Heart rate drops suddenly during running check pic below

Also had that twice now. Had watch since March. Overall it's pretty good at tracking, but every now and again there is that drop.

I think it has something to do with a hair or sweat in the way.

I also have such a drop.

BTW, before enlarging your pic I thought it dropped to nothing but I see it drops to ~70BPM.

Also in my yesterday MTB ride during ~140BPM session it dropped in the middle to ~85 and then recovered.

You are right, also the watch might be to loose or to tight. It differs from person to person.

There are many factors. In my opinion to rely on the heartrate, I´s suggest not using the wrist HRM. For allday usage it´s fine and simple (because it´s there). But for running or bicycle rides, I prefer a HRM strap. Got tons of material for comparsion, how far away from reality the wrist measurement is (30-40 beats higher on activities, drops, recovery hr only 3-4 beats, and so on).