I still don’t understand why it works in most cases when voltage is still 24V. In fact I tried my circuit many times at voltages of 30V where gate voltage is almost 30V and everything worked well

It isn't guaranteed to fail when the gate voltage exceeds 20V. It's only guaranteed NOT to fail if the gate voltage is less than 20V.
Sometimes you might get lucky.

 

That would in effect act as a voltage divider making your gate either 24V or 12V depending on the switch open or closed.

Indeed, that’s true. I wasn’t seeing the voltage divider. It’s the simplest and most cost effect solution

Thanks