We have to fully remove the display to investigate, but it turns out there's a new sensor facing the left-side hinge, and a magnet embedded in the hinge itself. The arrow on the magnet indicates alignment/polarity, which rotates in sync with the display.
Looks like Apple added a lid angle sensor. Why would they need that? Will this enable some cool new macOS feature?
Maybe. It could also be a clever way for Apple to trace the history of how the device was used if repairs are needed. For instance, if the display cables fray, Apple might want to know how much the display was adjusted, and how often.