I'm not sure how to fix this, but it's been happening since CC 2021.
Lightroom says I have 1 photo syning and it never finishes. When I click on "View" to see what it is, there are no photos to view.
I have tried rebuilding the cache, and uninstalling/reinstalling Lightroom CC but to no avail.
It's not really a hinderence, but it's very frustrating to never have a "fully synced" library.