So, this is not an entirely academic question for me - for a few years I've had one of two problems:
(1) My Nexus card renewal was "stamped through" in 2022 or thereabouts with no interview and, importantly, no new picture. A nice US CBP agent took a fresh picture for me on the way home last year, but it was still a repeated hiccup.
(2) My passport, which is nearing renewal (I can go for that later in 2025), isn't quite scanning properly - age and a trip through the laundry many years ago are to blame.

This has me legitimately wondering - "your face is your boarding pass" is being pushed by some tech dingbats (no, my boarding pass ought to be my boarding pass, otherwise why did the guy scan it on the way through security?). What happens when you've got a bad old picture in the system (whether because you've changed your appearance or because the picture is just lousy)? A lot of state DLs only require renewal in the 8-10 year range and a passport is good for a decade, so it really isn't hard to imagine a situation where I clear security on (say) the new Nexus picture but for some reason the system is trying to ping on something older/different.