Can anyone explain to me why Michael Bennett is listed as a DE in this league, but in another one of mine he is a DT? I'm curious, I thought MFL set the positions but apparently it's on a league-by-league basis?

I'd appreciate any clarification as I don't want one to randomly switch before the season starts

NFL does set them. But they also randomly change them. But your Commissioner has the ability to change anyone's position. You might want to see if he switched it.