I have about 110,000 photos and videos in my Photos library, synced to my Mac with iCloud with "download originals" turned on. Sync was working fine until a few days ago, about a week after installing Catalina.
I accidentally opened my Photos library from a networked Mac and closed it. It didn't appear to cause any problems, but the next time I opened Photos on the local Mac, it gave me a "repairing library" progress bar. That eventually completed and all my photos showed up. So far so good. But since then (five days ago), it has not downloaded any new photos from iCloud.
In "All Photos" view, at the bottom where it would normally say "Downloading x new items", instead it says "Restoring from iCloud...":
The numbers are wrong. When I first open Photos for just a second it shows slightly lower numbers, which are the numbers of photos and videos actually in the library. Then the "restoring" message comes up and it switches to a higher number, which is still lower than the number shown on my phone and other iCloud synced libraries. The photos missing from this library show up fine on those devices.
The iMac has been on and attached to a fast and reliable internet connection the whole time. I assumed it would resolve on its own, but it has now been 9 days and it's getting inconvenient to not have access to new photos.
When I posted this I was running macOS 10.15.1. I have since upgraded to 10.15.2, and the problem persists.
I've tried restarting and I've tried turning "Download originals to my Mac" off and back on, to no avail.
Last night (day 8) I opened Photos while holding command-option to rebuild the library. It took about 9 hours, after which it went right back to the same "Restoring from iCloud..." message.
Has anybody else run into this problem? Anything I can do to get it back to normal syncing?