I purchased my Active 2 about a month and a half ago. Everything was working perfectly until a few days ago. There was an update involved and ever since it stopped tracking my sleep cycle. Everything else is working fine. I searched the community forums and found that a few customers were able to restore this function after a hard reset. With few options I decided to do the hard reset. My Samsung wearable still does not register my sleep cycle. I hope a new update is available soon to fix this bug, a lot of people seem to be having the same problem.
Not much help, but I noticed that keeping the rear of the Watch clean, and the Strap tight helped to avoid the 100% Heart Rate Error issue. Not totally avoid, but this improved things. I used to wear my Standard Black Strap on hole 3, then went to hole 4 even though that was somewhat tight. Now it's comfortable, and the error happens less often. The Strap has probably stretched a little, hence why it no longer feels too tight, but the fit is better now than it was on hole 3.
I mention this because the Sleep Tracking is obviously linked in terms of the Heart Sensor on the back, so if you want to improve Sleep Tracking, then you also need to try and avoid the 100% Heart Rate Error if at all possible, or Sleep Tracking will be all over the place and no use at all.
I'm sure my Sleep Tracking is not that reliable. Often the Watch omits periods when I know for certain I was asleep, because I log my sleep very carefully for other reasons. So I end up having to add the odd Sleep Period manually (via the SAMSUNG Health App on my S10 Mobile), which is a PITA because that doesn't show me any breakdown in terms of REM Sleep, Light Sleep and the elusive Deep Sleep.
I also suspect that some Deep Sleep is not being logged, when I think I was hitting that target a few times.
I know this is no fix, but may help to improve things.