I’m not sure if this is due to PVS calculations, it being a screen-space effect, or whatever, but it seems that lens flares in the 3D skybox will not render properly unless you’re physically inside the 3D skybox and looking at their original position. The flares can render superimposed into the scene when the 3D skybox is being rendered as a duplicate, but it also has some odd offset that changes drastically when the player shifts their view.
I’m going to go out on a limb here and say this isn’t essential for the original BM campaign, but it would be nice if whoever wrote the renderer improvements could take a look at this and see if it’s at all possible to correct this given free time so custom maps can utilize them in the 3D skybox.
A sample vmf showing the bug can be found here.