I have had big heads mutator on and there are some moments when allies don’t have a big head and some of them do in the cutscene.
Jack only has one arm instead of 2.
But if you change the ability for the side that is missing Jack does change it when you go back but still no arm.
I have also noticed, i have played Act 1 on Inconceivable so far and progress is being done on my Seriously 5.0 achievement. Which i am guessing means even though it specifically says Insane for the achievement you possibly can unlock on Inconceivable