@AlexZ, it is unlikely to be the mosfet. Generally when they fail they do so permanently.
Is the new heater a high wattage heater (more than 40W)? Is your heat block a copper block rather than aluminum? In both cases I have found that standard PiD tuning is less than ideal and needs hand tweaking.
Also, the actual temperature graph may tell you more about what is happening.

@Phaedrux No. well below thresholds. The issue has been solved and noted in my above statement. Thanks

Assuming your bed heater isn't drawing excessive power and that you measured power supply voltage right at the power supply and not somewhere down the wire, you have a supply that is about to give out or a clone supply. Note that you can not tell a real MeanWell power supply from a clone unless you know what you are looking for or open it up. May I suggest you plan for a new power supply in the near future? You might see a 0.1 or maybe even 0.2V drop but most certainly not a 1.0 volt drop.
I do not believe that adjusting the output voltage will make any difference if your voltage swings all over the place.
This may not be the only issue but is certainly an issue in itself.