I've experienced the same problem with Fenix 8. Yesterday afternoon I took a nap (about 2h 40min) that replaced my night sleep. I've edited sleep times to bring back previous record. Now I have all the data as they should be, except the final sleep score, but there also wasn't any after a nap.

I'm also a bit disappointed with sleep recognition function. Very often I have to correct time of falling asleep, because it gets recognized much too soon. For example yesterday I fell asleep after 1 am, but according to Garmin it was ~2h earlier, while I was watching tv in my living room, so sleep score was lower due to too long time of being awaken.

I came here hoping, that there's a fix to that issue, or even a temporary solution, something that enables me to mark what is a sleep and what is a nap, anything. Instead I see, that this problem is known for years(!), and nothing seems to go forward with it. People here should at least get the answer, why is this problem so difficult to solve and what is currently being done with it. 

Garmin replied to earlier users' requests that this works as designed and they have no intention of changing it. I know it is dumb, as you as many users noticed, but that is how it is and will remain so. 

If it's "working as designed" then they should change their criteria because that's clearly wrong: in my case awarding a Mightycal sleep night for a crappy nap does not make ANY sense.

I agree. Deleting previously recorded sleep does not make sense at all. But as I said, Garmin has no intention of fixing it. 

Based on the ongoing issues with 7 Pro and 8 series, I highly doubt they have the ability to fix something without break something else.

Which on the other hand is very disappointing considering the price of these watches. I am yet waiting for 7 firmware to fix all the issues with the battery along with the sleep incorrect tracking and other ongoing issues.