For what it's worth, I noticed this when testing game slowdown in Temple of the Lizardmen II

See that slight shadowy curve across the sky looking a bit like an upside down grey rainbow?
The skybox in the map is constructed like this:

A central octagon(ish) shape with slopes creating a dome-like structure on which the sky texture is used.
And, like already mentioned, going back to an old Git build that I have lying around, the AO "rainbow" is not present.
However, it's also worth pointing out that, although in a skybox like this and the one from BoA, AO is not desirable across the sky, there may well be plenty of examples where it would be desirable. e.g. Something using an effect like Gooberman used to have a spaceship flying into a structure that was actually a moving skybox camera would look much better with AO in the skybox than without it. So it possibly shouldn't just be a case of "if the view is from a skybox camera, don't use AO" (if that is even possible). Or, maybe have some way of flagging skybox cameras so that they do "see" AO (defaulting to not seeing I suppose).