I have iOS 10 installed on my iPhone and macOS Sierra installed on my iMac. I'm using the iCloud Photo library on both.
On my Mac, I've spent a lot of time tagging faces and have done so on many of my ~35,000 photos. However, they do not seem to sync onto the iPhone Photos app. There, I'm supposed to do the whole tagging again, which I certainly don't want to do, given all the effort.
From what I've read so far, it seems that Apple has somehow not included this feature yet. Is that correct? Is there anything I can do about it?