This is a known bug if you upgrade to Catalina on your computer, all your photos that aren't in the root folder of photos disappear. So if you have your photos organized in albums, as most people do, they will not transfer. I called Apple Support got transferred to 4 different people who didn't know how to help and then disconnected. This is garbage. There have been so many issues with iPhoto and now Photos, upgrading libraries and syncing. As a photographer who needs to store and organize photos and have them mobile, I'm going to use a different service, likely Google Photos. After 30 years of Apple, they've lost my confidence.