I've used the Galaxy Watch Active 1 for two weeks now and every night it says my heart rate is somewhere between 30-45. I had Polar Ignite and Fitbit Versa 2 before this and they said 55-60 for my sleeping heart rate.
I am not an athlete so I don't believe my hr actually goes that low.
During the day the HR is fine, at 60-70 when resting and always spot on with my Galaxy S7 heart rate test.
The strap is firmly on my wrist one finger width from the bone.
Also according to the watch, I'm getting only a few minutes of deep sleep per night. I have tracked my sleep with Sleep as Android for years and I'm always getting a lot more than few minutes of deep sleep. Something like 1-3 hours. Also the other two watches showed more deep sleep.