Sometimes those little jumpers develop an OPEN connection under the rubber overmolding on the pin. It LOOKS like it’s fine, but it anything but fine. Looks like you have the PREMIUM jumpers though, so I’m not sure what it could have been.
Here’s a picture of my neopixel strip… notice the cap is right up close to the strip power input. This is ideally where you want it, but it’s arguably more of a pain to get it there. The reason is when the pixels turn on they will draw current abruptly and IR voltage drops ( current multiplied by resistance = voltage ) will form across all of your wiring and connection back to the point where current is sourced from. In this case it’s the big cap, for all of the instantaneous current requirements. It just means if the Core is powered from a tapped off point between the cap and the strip, it may see power spikes which could reset (brown out reset) the STM32 processor… this is probably not happening in your case, but these are just design principles which are good to know about and follow.
Anyhoo, glad you got it working