Lost photos face recognition after iPhone upgrade

Hi,

I recently upgraded from iPhone 10 Max to iPhone 13 Pro. I set the new iPhone up using iCloud back up. The issue is that now all my photos have lost their face recognition (I now cant search for people in photos).

I now have 64k+ photos to rescan and I've seen no progress in that number in 2 days. It just says connect to power and lock the phone....but then there is no change by the morning. Why doesn't it start working immediately and why were all the faces lost in the first place? Also, the faces are all saved on my Macbook Pro still so why on earth does the iPhone need to rescan? Can't the Macbook Pro not just share that info across?

I'd appreciate help if it can solve this problem. Thanks.