So I tested out the sensor a bit more. Shining a bright flashlight pretty directly at the sensor did get it to show 10,000+ Lux. If shone slightly less directly, it would do sensible things like respond with 1,500 Lux or something similar.
However, with ambient lighting in our room (by no means dark), it fell quickly back to 0. This is also what it ends up reporting on an overcast day where we don’t even need to switch on lights in the room. Far from the 50-100 Lux that it should be (https://en.wikipedia.org/wiki/Lux). Having all the lights on in the room brings it up to 12-13 Lux.
Seems to me like the sensor is just plain inaccurate and lacking in sensitivity on the lower end of the spectrum. I doubt a device handler is going to fix whatever issue it is having - but I hope I’m wrong. As it stands, I’m most likely going to try and return it.