Hi,
new information:
some are even discoloring due to heat
Discolouring means high power dissipation. But transients will not cause high power.
Therefore now I assume your power supply is failing. Maybe on overcurrent condition.
Imagine at overpower condition let´s say the voltage drops (maybe for a limited time) to 6V, then the MOSFET is only partely conductive, dissipating a lot of power.
A 30A power supply with 10 loads causing 4A each may be overload. (Especially when considering high starting current. Btw: did you check starting current?)
Switch mode power supplies are more sensible against overcurrent than linear power supplies.
I recommend to use a higher rated power supply.
Or include a power supply monitoring circuit, that shuts down load when the voltage is below a certain level. About 200ms delayed switch ON.
In any case I recommend to do some overcurrent tests and check the supply voltage with a scope.
You may use 10 incandescent lamps (30...40W) as load.
They will draw a peak starting current.
Now test what happens when all 10 channels are switched ON at the same time.
Then check what happens when the channels are switched ON, each with a 100ms delay from channel to channel.
Klaus