The voltage drop on a fully open STP55NF06L mosfet passing 12A should be 0.192 volts. To measure that directly you measure between gnd terminal of D8 and gnd of psu itself.
If you do get indeed a 12.4v on main connector and at same time 11.8 on D8, another probable drop (~0.4v difference?) could be on the big yellow ptc polyfuse, and may check that it does not get too hot because those are reported to catch fire.
Perhaps the entire psu voltage is 12.4v without significant consumers, drops to 11.8 when the heatbed is on, and jumps back to 12.4 when the heatbed is off. Because the bed is a big consumer. I'd rather think its like this instead. Meaning you get different voltages, but at different times depending on the bed on/off state. Other than that, inside either state the voltage differences should not be that big.
If you do get indeed a 12.4v on main connector and at same time 11.8 on D8, another probable drop (~0.4v difference?) could be on the big yellow ptc polyfuse, and may check that it does not get too hot because those are reported to catch fire.
Perhaps the entire psu voltage is 12.4v without significant consumers, drops to 11.8 when the heatbed is on, and jumps back to 12.4 when the heatbed is off. Because the bed is a big consumer. I'd rather think its like this instead. Meaning you get different voltages, but at different times depending on the bed on/off state. Other than that, inside either state the voltage differences should not be that big.