I had the same issue.
I decided to give it a try and turn iCloud off and back on again, which helped and now scanning resumed and completed after about an hour. Before this, scanning was stuck at 700 pictures (of 2500+) for a week although it was on a charger and locked every night for hours!
I have the latest public beta, which is the same as the official iOS 10. I installed it on the day it was released, 25 days ago.
One weird thing I noticed was that although iCloud backup was always on, when I wanted to turn off iCloud, I got a message which said that no backup had been made for 58 days. Maybe the bug is somewhere around iCloud backup. I used to have backups to the iCloud every night and for some weird reason it stopped 58 days ago. I just don’t know why.