It's a lighting issue, a technical issue, and a subject issue. It's just a bad combination of an object that can change head position faster than the depth map can update, and flat head-on lighting.
It is what it is, same reason you can't really get great macro flower shots because twigs and branches are everywhere and the complex background of additional plants will fool it every time. You can try using the Focos app's "EFFECT" tab to dial in where the depth starts for both front and back bokeh on a particular image. I've been able to save dodgy depth edges around hair and other complex edges this way - you can basically set it to begin blurring the map further back.
I also think a subtle wiping/waving motion of the phone around your intended frame before firing the shot will let it grab a more accurate map, in the same vein as AR apps needing a decent look at the room before nailing down a solid tracking.
