Yeah people fail to remember that the Jungian type has an Enneagram type attached.
Whatever baggage there is on such a level, will be expressed nicely there.
From the OPs words the Enneagram of such a "bitter" ISFP could be many things.
Personally I'm probably a 5w4 9w8 2w1 Sx/Sp.
That combo combined with my experiences gave me plenty of bitterness towards the world.
As a child I was just like people like to describe ISFPs, but now I'm more on the cold cynical side.
I was even more bitter before, I've managed to deal with some of my baggage,
but I have enough bad experiences to draw on to chill any conversation should I wish to.
People are dynamic and can change when the world force them to.
I think many would label me as an INTJ to avoid dealing with the dark shit that mar their innocent picture of ISFPs.