Same here with vivoactive 3 Music... is there some solution?
So...weird thing. I recently got an update abut 1 week ago, I just purchased that Garmin two weeks ago, but whey I've noticed is, the initial sleep syncing does not show REM, but later in the day (about 1pm-5pm) if I look back at the app, the REM sleep will appear. So maybe the Garmin app/watch just needs more time to calculate/adjust your sleep to to REM sleep? When I first wore this watch however, REM would show up immediately with light/heavy, awake, and REM for sleep, so I am not sure why this recent update changed this?
For those experiencing this issue, this should be reported to Garmin via their support site (phone, chat, or email) to get a case opened or have yourself added to an ongoing case number. Although this forum is useful for user-to-user assistance, it is not necessarily the best method for getting Garmin support involved and working on the issue.
For me after 1-2 weeks the problem disappeared and everything worked. It's hard to say why this is happening in the beginning.
I have no REM sleep detected
either