One of the advantages of a 230V bed heater....
300W heater @12V is 25Amps @24V is 12,5Amps
Low voltage is considered safe, but the currents are not!
25 Amps requires thick wires and high current connectors.
If the connection is not ok, you will have a resistance at that point. A resistor@ high current will get hot and the consequences are clear.
Current for the same 300W with a 230V heater is only 1,3 amps.
Another point is that the FET has to switch high currents with almost no cooling. Sooner or later the FET will burn...
When using these high current heaters I strongly suggest to mount a well designed FET with cooling off the board. These MKS type boards are not well designed for that.