I have a simple strip of 120 ws2812b leds connected to an arduino nano powered by a separate 5V supply.The supply is rock solid on 5V. I am using the fastled library.
I am using a MOSFET switch controlled by the nano to power them up or down.
There is no data transmitted to the leds at power up so they are all effectively off.
I have had a problem with random flashes and strange behaviour on power up.
After trying many capacitors to help I started looking at the power up rate. I have an oscilloscope.
The MOSFET switches the supply almost instantaneously (micro seconds).
I started slowing it down using some capacitors on the MOSFET gate. As I did the strange behaviour reduced.
Now the leds are powered up over 5 millisecs and the leds do not flash or glitch. This might be overkill and a shorter time may work but 5 miliisecs looks good. Data can then be sent and the leds be controlled.
The second discovery is their susceptibility to EMI. I have a washing machine motor nearby I ended up changing the power/data lead to a shielded cable and again my problems were solved.