I received my cubey kit yesterday and have been trying to set it up. With the provided LED strip I’m lucky if I can get 17 lights to illuminate, but it’s often around 10, with several of the lights afterwards “stuck” in their previous state. Which isn’t great when the last command was to turn off all the lights.
I managed to find an old neopixel ring which has 16 LEDs and that worked fine with the Plasma 2040 W.
I’ve trimmed the cables at the start end, use different USB power sources and different USB cables.
What you see is more of a signal integrity thing, and less of a power thing. Regarding the latter you also already did what is possible to try. And your successful test with the neopixel ring clearly hints that your LED wire is the problem. I think it is ok to mail support and ask for a replacement.
I agree with what Bernhard said. I have two of the kits that I use with mini Christmas Trees. None of the issues your reporting. I do really like those LED strings. Bummer yours doesn’t work. Contact Us for Raspberry Pi Technical Support - Pimoroni