I have a decently amount of photos in my online collections (about 500k). A few days ago out of nowhere changes and edits I was making in Lightroom Mobile/CC on my laptop were not being pushed to Classic. I could see these changes being made on the web app and other lightroom CC devices, so the problem seemingly lies somewhere in Classic's ability to receive them.
These collections are just made up on the smart previews generated from uploading to the cloud via classic. The bug completely destroys my mobile editing solution. I'm a wedding photographer.
The sync bar at the top stays green the whole time. I just got done rebuilding the sync data (took a few days), and it fixed nothing. Doing it again now out of desperation after having reinstalled lightroom classic.
I can see other people having what sounds like a similar problem in these forums with no solution. Example here.