OK. I’m guessing that the number of Photos (= photos & videos) of not unable to upload to iCloud Smart Album is now equal to the # of photos and videos in iCloud.com Photos. In fact, I’m guessing all of these #’s are equal:
- Photos and Videos at the bottom of the Moments tab in iCloud.com Photos
- All Photos # of items at the top/right under the Photos tab in iCloud.com Photos
- Photos Library # of items at the top/left under the Photos tab in macOS Photos
If so, it seems possible you may have the same problem albeit different manifestation as discussed here.
https://discussions.apple.com/thread/7785024?answerId=31102686022#31102686022
Ok, so I am not sure what is happening; however, I had determined that there are a lot of duplicates and some of them there is no information in the exif data anymore - just shows no camera information available when the exact same photo further up in the all photos list has all the data. I am not sure how this has occurred but it seems to have.
I have gone through and exported 1000 block photos at a time and when the numbers were not matching, made the blocks smaller until I found the exact row of 10. One example was, I selected 10, said export and it exported 8 of them. I then went through and exported the exact same row again, image by image and it did all 10. So there seems to be some sort of bug in the Photos.app; however, I can't determine what it is!
The author identified some kind of transient false duplicates in macOS Photos library via process of elimination. You might try to do the same i.e. count or select all for one given time frame after another but simultaneously in both macOS Photos as well as iCloud.com Photos until you find a discrepancy and narrow it down until you find the actual discrepants. If/when you find some discrepants, see if they aren’t false/erroneous duplicates and/or look for any other common denominators e.g. file type et c. Be careful selecting massive quantities of Photos all at once. One false click from there and who knows what could blow up.