The first generation of phones with in-display finger scanners, like the Vivo NEX with its pop-up selfie camera, carry under-glass readers which are of the optical variety. Qualcomm's new ultrasonic tech can work accurately through glass up to 800 microns thick, compared to 300 microns in the previous generation. The optical variety is typically supplied by the Chinese firm Gudix whose solution is the one currently used by phones with in-screen finger readers.
That's not to say that the OnePlus 6T will have trouble with unlocking, as even the optical scanners have proved to be perfectly operational even in their earliest incarnations. They work through stacks of screen protectors, too, and are just a tad slower and a bit less precise than the ultrasonic ones.
At this point, however, we'd take anything over the old argument if the finger reader should be on the front or on the back, as the in-display readers render those points moot on today's "bezel-less" phones.
source: OnePlus (Weibo)