Controlling IRF540N with MCU for LED on/off switching
Do not disconnect a ground of a device that has data connections. That is a sure way to damage your MCU data pin with overvoltage and overcurrent.
When you turn off the FET, the chip inside WS2811 loses the ground connection, and if data pin is low, current can and will flow into WS2811 supply pin and via WS2811 internal data pin protection diodes out of data pin into MCU data pin and from there to GND.
In case of WS2811 has a larger supply voltage like 12V, current can flow into MCU data pin even if pin is high, and it actually tries to power the MCU via internal protection diodes.
The circuit diagram internals of your drawing and how WS2811 actually works, so it may provide a wrong sense of how it works.