ws2812b 5V power up rate and noise affecting the leds with solutions

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.

Comments welcome.

There is no data transmitted to the leds at power up so they are all effectively off.

No, sorry, as I understand it, they will be completely random! And if you deny them power with the MOSFET, they can't receive data, I suspect.

I do not see that behaviour.

My experience is they normally power up blank unless the supply is applied with a high slew rate then the leds may well be random.
Secondly I found the leds may not respond to the data if powered rapidly. They appear to be in a fault state that only a power reset fixes.

My finding is to power the led with a slew rate over 5ms and then supply data to control the led.
The led will start blank and then be controllable.