Just ran into this with a number of SK6812 RGBW strings I have. These are all pre-soldered with 10cm of wire between the LED’s. They are simply bare 1cm PCB’s with a SK6812 on board and soldered together with 3 wires in and out.
Works great on 100% master and segment brightness. If I tune down the master or segment (or both) brightness then trying to set more than any 2 of the RGBW sliders to other than 100% or 0% it causes the strip to flicker and flash.
I have done this with as little as 10 LED’s assigned to a segment and it doesn’t matter if it’s the 1st, middle or last 10. I can achieve the brightness I want by leaving the master and segment at 100% and simply dialing in the effective brightness with the RGBW sliders and that seems to work OK.
My power supply is a 5V5A and wired directly to the ESP32 and the strip, injected at both ends. Worst case measured voltage was better than 4.8 volts on the strip and 5.0 at the ESP32. Distance from the ESP32 to the string is 4cm.
I verified this on a SP511E device (latest firmware) as well as release 0.12 and 0.13b2 on an ESP32.
Further evidence that this is a software incompatibility is that I also used the same ESP32 device on a 40 LED RGBW ring and a string of sealed epoxy SK6812’s “nodes” that are spaced at 10cm. Both those strings work exactly as expected and allow full brightness control from master and segment.
My rough guess is a quirky timing issue with the bare string LED’s.