Really obvious high heart rate notifications not working?

Hi. I've had a series 3 for about 3 years, I bought it because I wanted to keep a check on some irregular heart rhythms I was experiencing (possible Afib). For a couple of years it was regularly registering high heart rate notifications whilst I was sleeping. I would go from my regular 60BPM resting heart rate to 140BPM or higher, the watch would record this for 30 - 60 minutes, sometimes more than once per night, before returning to my normal resting rate. A corresponding high heart rate notification would also be recorded.

Since May 2019 no high heart rate notifications have been recorded despite the heart rate recorder showing these heart rate spikes perhaps once per week.

Yes, I have checked all the settings and been through the heart rate notification setup repeatedly. There is no obvious reason why this feature no longer works. My notification threshold has been set to 100 and also 110 BPM, heart rate has been higher than 120 whilst sleeping recently but still no notification!

I've tried a full reset and reinstalled apps on my watch, hard reboots on both watch and iphone (I'm using Iphone 6). Both watch and Iphone are fully up to date with IOS. I'm at a loss, it's just not working.

As a side question; I've read that it's possible for Apple watch to read BPM incorrectly due to movement during exercise etc and the reading is usually low. Is there any possibility that the BPM could incorrectly read high? I'm still not sure these heart rate spikes I'm getting whilst sleeping are genuine. Whether genuine or not, Apple watch should still record the high heart notification.

Cheers

Rob