Discovered that, on a MY2017 car:
It has incandescent bulbs in the rear light clusters.
If one of the brake light bulbs fails, there's no bulb failure indication.
There are 2 brake light bulbs in each cluster, though, so no drama, surely?
.. Except that it disables the working one when it detects that the other one has failed.

Filed under "words fail me".

Mind you, they couldn't make the clock work properly either, or the sat nav.
