I installed a new SSD drive in a Mac Pro 2009 (with 2010 firmware). I partitioned the drive and did clean installs of Sierra (HFS+) and High Sierra (APFS). I did not migrate anything from any other system.
On both partitions I enabled System Preference > iCloud > Photos. I selected "iCloud Photo Library" and "Photo Sharing" in System Preferences > iCloud > Photos and in Photos > Preferences. I also selected "Download Originals to this Mac" in Photos > Preferences.
On both partitions, all shared albums instantly appeared in Photos, and Photos gave a count of all the images it had to download from iCloud.
On the High Sierra partition, Photos proceeded to download nothing, even after several hours. Not even low resolution images.
On the Sierra partition, Photos rapidly downloaded low resolution images, and then very slowly started downloading the high resolution versions.
I took a photo with my iPhone and another Photo with my iPad (The iPad is running the latest version of iOS 11. The iPhone is running the latest version of iOS 10. Both have iCloud Photo Library enabled). Both photos appeared on the iPhone, the iPad, an El Capitan Mac, and the Sierra partition. Nothing on the High Sierra partition.
I copied the incomplete photo library from the Sierra partition to the High Sierra partition. Photos gave me a message like "2,266 Photos, 16 Videos, Downloading 2153 Originals” and proceeded to download nothing until (after a couple hours) I tried selecting images one by one to force Photos to download high res versions.