SK6812 - LED Flickering
As your LED_DATA trace gets longer and more LEDs get added on, the capacitance of LED_DATA will increase. It's likely that in combination with the 470ohm resistor, you're creating a lowpass filter and rounding your LED_DATA signal. As the capacitance increases and the signal gets rounder and rounder, eventually it will stop working. If that's the case, replacing it with a jumper would fix it. Try looking at the LED_DATA signal (with and without the resistor) with an oscilloscope.