Coincidentally, I noticed exactly this phenom using the ef24mm f/1.4L ii last weekend. The room was decorated for Christmas with those icicle lights hanging from the ceiling.
After reading this thread, I wonder if it is not the lens at all, but a different issue of light. When you look directly at any light bulb, it has some light source element inside and an outer casing (glass or plastic). Light passing directly from the source to your eye (or lens) passes through a minimal amount of the casing. Light at the sides or border is passing through substantially more casing material and is greatly diffused.
Anyway, just a thought; could be completely looney. But if there is some "fault" in optics, I want to lean toward the maker of the cheap light bulb rather than a team of optics engineers who designed and built a very expensive camera lens.