hr sensor always blink

This is my experience also. If you power cycle the watch the lights are off until you put it on, thereafter they stay on. No activity recording required to trigger the errant behaviour.

yep, with en emphasis on "at least"!   Mostly more than one though. And they recycle the bugs.

by far the best solution I I have heard. Garmin knows  most people will put up with this crap. I say buy/return every week. :)

you are wrongly assuming Garmin has QC. Your best option is return the device.

Recycling the bugs  

https://forums.garmin.com/sports-fitness/healthandwellness/f/vivoactive-4-series/206292/hr-sensor-never-stops-after-turn-off/969005?focus=true

I have the same issue. Garmin replaced my Vivoactive4s but the new one has the same problem.

Restarted it does not really help. The sensor blinks like crazy again after recording an activity.

It helps: you won't get drained between activities. 

That's not the point. It's bug and must be fixed. Users should not have to restart watch after each workout.