Random chunks of photos not being uploaded to icloud

Hello,

I’ve been trying to fill transfer my photo gallery from my iPhone 8 to my iPhone 12. I backed everything up to iCloud from my 8 yesterday morning and restored from the upload on my new 12 soon after.

However, today I noticed there were a bunch of pictures missing, and in weirdly specific chunks too. Out of my 28.5K photos on my 8, only 23.5K uploading to iCloud. I’m certain this is a problem with them uploaded to iCloud and not a problem with them loading onto my 12, since my iCloud gallery on browser also says it’s only 23.5K photos.

The missing photos are in weirdly specific chunks as well. For example, my iCloud September 2020 photos stop after the 12th, despite the fact that I have pictures from all through the month on my 8. Same goes for many other months, with the later half’s photos just missing?

Any ideas on how I can make sure all the photos from my 8 upload to iCloud?