Just an informational post in case this helps anyone. Be cautious if you rename the system library or change the library from being the system library (and then change it back).
I have two Macs and had a local library on each Mac with iCloud Photo Library enabled and everything was humming along. After realizing that Projects like Photo Books don’t sync over the cloud I decided to use an external hard drive to store the main system library. I figured I would just use that library on the external drive instead of having a local library on each Mac. So I copied over one of my Photo Libraries to the external, renamed both local photo libraries to “old” so I didn’t confuse them with the external one, then made the external hard drive Photo Library the System Library. Then I changed my mind and decided I’d prefer to have two separate libraries on each local machine and just use one computer for photo books so I simply renamed my “old” files back to the default name and made them each a System Library. When I checked the checkbox to use this library in iCloud it told me I needed more iCloud space and after I clicked through it prepared and then began “uploading” the photos. These photos already exist in iCloud so apparently they aren’t really being uploaded but just reindexing information. I called Apple Support and that’s what they told me. The support person said that changes like renaming the library can make iCloud think that it’s a new library and therefore needs to check every photo to make sure that it’s on iCloud. I verified that it isn’t duplicating photos by logging into my iCloud.com account on the web browser and looking that the Photos/Videos count remained the same. So not really a problem per se, but more of an annoyance. I wanted to share in case anyone else ran into this.