Primarily with Beta Timeline
I have personally found the sync behavior to be inconsistent following 1.139.3, which I found to be the most stable release. Ever since 1.140 and beyond, the app doesn’t consistently refresh when you open it and is more likely to have issues with thumbnails not loading.
These issues are always resolved by a simple swipe away and re-open of the app.
I’ve tried signing out and back in and uninstalling/reinstalling the app a couple times.