Yes, this is it. I have the Galaxy Watch and my heartbeat sensor works flawelessly, came to this thread because my friend bought it and his AOD didn't work properly (only rarely), so came along this reply and yep that is it. Btw. we tested it on my wrist aswell, in the exact same position I wear my own Galaxy Watch and ofc it didn't work.
HOW TO FIND OUT FOR SURE - you might need to try this several times, possibly at different time of days or when you see your display is off.
Go to the Heartrate App - change the heartrate frequency to anything but always (so sometimes/often and never). Now wear the watch and click "Measure" - if it says now the sensor needs to be cleaned andor you should wear it in a different spot, it is a good indication that indeed it is a broken heartbeatsensor (but try to clean it, wear it where it says you should and for testing purposes look that it is tight - and then test it again).
Additionally what you can do: Some of the watch faces have a real time heartbeat number if the display is FULLY on. If it doesn't change numbers at all, then you might look at a dead heartbeatsensor.
Now me and my friend are gonna return it and get a new one. (maybe later we will completely refund due to compatibility issues with iPhone X - it often loses connection and you can not update over the iPhone, wtf)!
She is lying to you, I have a Galaxy Watch, paired with Samsung Tab S4 and it still does work as it should.
I am on the newest OneUI update.
Check if your heartbeat sensor is working as it should.
I've got my watch yesterday and installed the latest upate, AOD is working fine. I wanted to use NATO straps with the watch, but that indeed disables the AOD because the heartbeat sensor doesn't have contact to the wrist. Well, there are always split straps.
I couldn't get mine to work and I tried everything. Then I moved the watch slightly up my arm (not directly on my wrist and it works now) problem is tied to the heart rate sensor.