Photos uploaded from Windows to iCloud Photos do not appear on iPhone. Why?

My wife has an iPhone, and has 40,000 pictures in her iCloud photos. Now she wants some older ones, that are on my Linux fileserver. My Windows 10 desktop has iCloud for Windows installed, and logged into her Apple ID. I can see her iCloud Photos in my iCloud Photos folder, and I have imported photos from her iPhone by dragging them into folders on my P: drive, after whcih I can process them from the Linux command line. On Windows, my photo library is mounted as drive P: (for pictures). When I drag photos from my photo library to the iCloud Photos folder in File Explorer, they show up in iCloud folder. Sometimes they can then be seen on her iPhone, other times not. I can't really see what the difference is. One set that worked will and comes up on her phone is from June 2018. One set that does not show up is from August 2016. The 2016 pictures are from a period when she had suppressed geo-tagging, but they do have GPS timestamps in their Exif metadata.