Makes me think there has to be some kind of wiring loose or not fully connected there at the topper. The light in question is definitely getting power, as the light is on, but since its not "interacting" with the rest of the neighboring lights, I would trace the wires from that light (there at the topper) to whatever it is it's next connection up there. My guess would be a problem at that next connection. Hopefully it's something that stands out as obvious.
Have you taken the topper down to explore the back of that topper and it's wiring?

Is it one of those LED boards associated with the light thats not working properly? I've seen wires poorly soldered to these small boards before. So if so, I'd unscrew that board and take a look at the back of it and see if one of the wires is loose/needs to be re-soldered. Can always consider exchanging for a new LED board if it does look ok, just to see if that works/current board is for whatever reason bad, which would be unusual.