How can I force the Photos app on my iPhone to recognize faces I have a several photos in my camera roll, received mostly through WhatsApp in my iPhone XS. 

In the photos, the person is in front of the camera and the face is completely visible and straight, not tilted, and when I swipe up to tag the person in the photo, I see a "No related found" message, it's not like it's mis-tagging, it doesn't find a face at all. 

Selfies taken with the camera and photos taken from other places (downloaded from Facebook, for example) seem to be fine, the face is recognized, but Photos doesn't seem able to recognize the face in these photos from WhatsApp.

I have some photos of this person tagged and going to Photos > People to confirm additional photos doesn't find any additional photos. 

Is there a way to force Photos to perform face recognition across the whole camera roll?
The only way I know to force it perform face recognition is to open Photos, Lock the iPhone, plug it to the charger, and let it locked and plugged for some hours.