Same issue for me for last week, no sleep data at Garmin App. Any news? However, at Garmin Connect Web Page I can see that the watch has detected the time of falling asleep and wake time during these days, but says "no movement activity during sleep" so sleep is not correctly tracked: no zones of sleep, no data.
I had this issue last week. No sleep recorded for 3 days. What I figured out was that i hadn't saved an activity on the watch from 3 days ago. I hit save. No sleep data on Garmin connect, So i restarted the watch and closed the app on my phone. After the watch started it pushed the sleep data to Garmin connect and all 3 days were visible.
It might have just been the restart and auto-sync that did it. If that didnt work i would have returned it for warranty.