iCloud Upgrading to iOS forces all Photos faces to be re-scanned?

[temende]

I've been using iCloud Photos for almost 5 years now. I upgraded to iOS 15 a couple weeks ago and for some reason it wiped all of the indexing of faces across my photo library (~20k photos). It's now been re-scanning for 2 weeks but still not close to being finished. Does anyone know why iOS 15 needs a complete re-scan of my entire library? Why can't it use the photos data from my previous iOS installation?

 

[kaardowiq]

Face scanning got improved massively with iOS15. Now, it recognizes faces even on side shots or in the background. Therefore, pictures are being rescanned. My 40k photos took just a night... Make sure it's connected to a charger and not used.

 

[temende]

Strange then, my phone has been plugged in every night for 2 weeks and it still hasn't finished.

 

[kaardowiq]

Do you have larger videos in your lib? It also scans for faces within videos which may take much longer. Also older devices may need more time.