
This is not the first time this happens. How to fix??
If it continues to happen, then I recommend to Remove and Add a Device From the Garmin Connect App | Garmin Customer Support, then log a few minutes of steps and HR, and record a test activity, wait till the next morning, sync, and see whether it helped. If not, then have a look at the Troubleshooting guide for (some) data missing after syncing for more tips.
Unfortunately the removing/adding will not restore the lost data, but it prevents it from going on to lose more of it.