I'm in the process of discussing this with Garmin Support but I'm wondering if anyone else has this same problem. Both my wife and I have vivosmart 4's, purchased at the same time. All our syncing with garmin connect is done with garmin express. Since the upgrade to 6.20 I have noticed the issue that data such as fit files from the previous day is not being uploaded, even though it is being saved in the watch. Here's an example, if I sync at 5 pm my time, and then sync again the next day at around the same time, my data from the previous day from 5 pm (the previous sync time) until midnight does not appear in garmin connect.
That being said, if I use the import data function in garmin connect it pulls in the unsynced data from the watch so it's obviously being recorded by the watch, just not getting "there". Similarly, I have another computer, totally different hardware, still running garmin express 6.16 that I've never upgraded, and it pulls in all the data properly. My gut feel is that it is an issue with 6.20. If anyone has any insights on this issue, fire away. If garmin can't correct the problem I will probably just downgrade to 6.16 on both machines.