It might be premature to conclude this is fully fixed but since installing the new beta (8.99) the reported stress levels dropped quite significantly for me (from ~20 during sleep to ~10 during sleep). Those were levels I last saw before 7.60.
I agree they fixed something in 8.99 beta firmware. For the last few months I did not get any rest periods at all, even during sleep. Granted this year had a stressful beginning for me in general, but that what an average day looked like (stress score around 40-50), see pics attached. And last week I had pretty bad flu/fever, so the score went up almost to a 100...so it still worked in a sense that it showed me a trend, but no restful period during my days off seemed not right. I updated it to 8.99 last night, and during last night sleep I woke up and looked to the stress widget out of curiosity and it gave me 0 - rest! I have never seen 0 before, so it all makes sense now!