In context of FaceID with screen protectors... herewith is a technical explanation of the both issue and its resolution...
The FaceID system has at least two seperate but critical optical components - these being the IR Dot Projector and IR Camera elements. These elements are combined to produce a single optical sensor system - and it is one or other (or potentially both) of these elements that suffer performance issues if covered by a glass element.
Even if the two outer surfaces of the glass screen protector are totally parallel (any deviation from which will cause defraction), the covering glass layers, optical coatings and necessary fixing adhesive will all have properties that influence absorbtion of light at certain optical frequencies/wavelengths. Additionally, any change in material density (such as glass-to-air or glass-to-plastic surfaces - including internal lamination layers) will introduce internal reflection and scatter at each layer boundary. It is these optical properties and their interaction with the sensor system that are likely interfering with indended functioning of FaceID.
The simple solution to FaceID issues, in context of applied screen protectors for both iPad and iPhone, is to select a screen protector that does not cover any of the active sensor elements (i.e, the screen protector will have a corresponding notch or cutout for these critical elements) - this being a feature similarly necessary for correct functioning of TouchID (which is also a different form of optical system).
It has been suggested by some contributors that “film type” screen protectors have been successfully utilised. Given that the film-thickness is considerably thinner than that physically possible with a glass alternative (also noting that plastic film lacks laminations), it is possible that the critical sensor system suffers less optical performance degredation.
Any issues with screen touch sensitivity are completely unrelated to the optical or mechanical properties of the screen protector, but may be influenced by dielectric characteristics of the appled screen protector material(s).