It also seems to depend on the scene.
If you put a wall “behind” the light, it works, if you remove that wall again, it breaks again, regardless of the inner cone angle.
Issue 1 (still working on the images for the 2nd.)
Setup:
As long as the back wall is there, the GI works:
If you remove it, it completely culls the light:
Issue 2:
Setup (see inner cone):
As soon as you move closer to the wall, the light gets culled:
But this does only happen with narrow Inner cones.
Putting the “Inner cone” at 20, it works again:
I am not sure if this is 1 or 2 bugs, but there certainly is something broken.
Luckily, for most cases the workaround of “having an inner cone” should work, for now. (coincidentally, thats why I couldnt replicate it, giving spot lights an inner cone is pretty much the first thing I do after bringing them into the world, for visual purposes.)
Feel free to add this to your bug report, so that epic has a “starting point”.
EDIT: My thoughts:
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Could it be that the “outer cone” only does screen space GI, while the Inner cone does proper traces? (that would explain Issue #2)
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the wider the cone, the more GI I get in the above setup “behind” the light.
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Lumen “breaks” when “looking into the void” (the removal of the wall is exactly that, and it doesnt appear to happen if there is another room behind the wall.)