iPhone Photos stuck face analysis iPhone Photos is not finishing analysing faces. It says : 


  6974 analysed photos. To finish analysing remaining photos (18002), lock your iPhone and plug it. 


(Translated from French). Following these instructions doesn't help. Nor does rebooting or closing Photos and restarting it.
The device will analyse photos when the device is locked, connected to power, and crucially when it thinks the device won't be used. Plug the phone in overnight and it will analyse photos while you sleep. There currently doesn't appear to be any way to force analysing.