I had the same issue on the watch 4 dips in the 60/70s I got a referral to a sleep apnea clinic and my results are fine. Just another gimmick and clever marketing these smart watches are not that smart, which is why I never upgraded, absolutely no point its just a glorified step counter with some bells and whistles.
Edit:- For transparency I think the bpm is fairly accurate but so is a Chinese bpm/spo2 meter that you stick on your finger and those are cheap as chips
I posted a similar topic a few months back, I always wear my watch 5 and 4 prior 247, I was snoring loudly after seeing a consultant and have and overnight full on spo2 device, he said I never had a single episode, that said my Watch Pro was say I was regularly 15 plus minutes below 70%.
So is the watch 5 pro a waste, err no, like the BP, BMI, etc... its a useful directional trend indicator that's about it.
I said that as after having an operation for a deviated septum, I am now always towards 99 to 100%, maybe minor episodes, but I think there is a clinical definition of sleep apnea and time below that is regarded as and issue or not.
DAILY DEVICES ZFold4 [Mobile], Pro5 [Watch], S8 Ultra [Tablet], Book2 Pro i5 5G [Laptop], Buds Pro2 [Buds] M70A 32/43" [Monitor], Smart Tags+, iPad Mini 5 and iPad Pro 12.9 GEN2 [Tablets], Dell Inspiron G5 Laptop
HOUSEHOLD DEVICES ZFlip3 and ZFlip4 [Mobile], Watch5 40mm and 46mm [Watch], S7+ and S6 Lite [Tablet], QN95A [TV], Q900A [Soundbar], MC28M6075 and MC32K7055 [Microwave], Jet 75 Complete (Vaccum), Samsung Level [Speaker], Buds Pro [Buds], iPhone 12, iPad Air 2, Air tags