My guess is the large FET but but technically it could be either one since we have not gotten a verification on which one was the problem from some of the others.

It does not make a lot of sense for the small one to be the issue though as it only has to deal with a small amount of power and even a cheap FET should be able to handle the load.

I have not personally seen this issue so I have not been able to test it myself to figure out which one it is. I do have one light with the stock driver but I swapped the big FET and have not had any issues otherwise.