Photos app apparently re-uploading everything to iCloud

See this article iCloud Photo Library - Apple Support

Pause library uploads to iCloud

When you turn on iCloud Photo Library, the time it takes for your photos and videos to upload to iCloud depends on the size of your collection and your Internet speed. If you have a large collection of photos and videos, your upload might take more time than usual. You can see the status and pause the upload for one day.

• On your iPhone, iPad, or iPod touch, go to Settings > [your name] > iCloud > Photos. In iOS 10.2 or earlier, go to Settings > iCloud > Photos. You can also open the Photos app, go to the Moments view, and scroll to the bottom of your screen.
• On your Mac, open the Photos app. Select Photos in the sidebar, then click Moments in the list of tabs in the toolbar. Scroll to the bottom of your photos and click Pause.

I'm still not really sure I understand how Photos is doing it's calculations though. At one point yesterday, Photos reported "Uploading 21,710 items (619.1 MB of 4.06 GB). However, once it got to 4.06GB, it just continued to report "Uploading 21,710 items" but without any indication of how much data it is expecting to transfer.

It is not documented, how Photos is presenting the numbers for uploading. I have also seen them increase and decrease again. As long as there is a kind of progress, be patient and don't interrupt it.

, it sorted itself out and it didn't end up re-uploading my entire library

We do not really have a way to tell, if the complete library has been uploaded again, because Photos is comparing the photos to the library in iCloud and solving conflicts between duplicates. I could not find any documentation on how this duplicate detection is performed, bt occasionally I am seeing messages in the Console about duplicate detection in iCloud. If your library does not have any newer photos not already in iCloud, the number of photos will remain the same, because duplicates will not be added. Here is an older screenshot of the Console, when I had to upload my Library again:

As I said, I could not find any documentation on how Photos decides how many photos need uploading at a time, and why these numbers keep changing. We are not allowed to speculate in these forums.

there are many possible reasons and obviously only you know what has been going on with your device - any this that disconnects and reconnects your library to iCloud will trigger a new upload to verify library integrity - these events include re-namming the Photos library, moving the Photos library, restoring the library from a backup, logging out of iCloud and logging back in, turning ICPL off and back on, etc

once you trigger the verification upload there is no alternative except to allow it to complete

LN

The figure will sometimes increase or decrease but you must be patient , simply open the photo application for a day or two , network speed is essential , and let the process be completed , your data could be massive so it will take time .

Thanks for the reply. The strange thing though is that nothing apparently changed on my end.

Photos will also upload the library again, if I needed to repair it because of a corruption. Thsi can happen, after Photos crashed, or or if you used the library from a different user account.

Is your Photos Library on a compatible drive? Are you keeping the library in your Pictures folder on your MacBook Air or on an external drive?

You might want to turn off Little Snitch until the upload completes.