For what it's worth, I'm sitting here running Catalina 10.15.1 sync my 11" iPadPro and iPhone Xs at the same time. If you put the iOS photo browsers in "All Photos" and scroll to the bottom, I can see "xxx Photos, xxx Videos" and "Syncing..." If I compare those two xxx values with the Photos app, I can see both iDevices slowly creeping up to parity with the MacOS library.And every minute or two you can see the thumbnails dynamically update as the data comes in.
But I'm plugged in the whole time this is going on and the Finder is giving me no indication that it's doing any syncing at all. In fact both iDevice icons in the sidebar are only showing the eject icon...no preparing to sync or active syncing feedback of any sort.
Further, my iPadPro seems to have stopped at 20,698 photos and 173 videos, as my iPhone continues to climb past both those numbers on the way to 21,858 and 192 respectively.
So it appears that syncing does work while the devices are plugged in, but there's no visible clue in the Finder. And then there's the iPad issue stopping.
Maybe this will be of help to everyone out there. And for just FYI, my library has nested folders...everything seems to be working.
Um, and now my phone has stopped short of the goal. Hmmm....