I believe long term abuse or neglect can scar people. I have been told so by a neurologist, which is why abuse is bad and bullying (including verbal abuse, relationship abuse, and cyberbullying) leaves life long wounds.
I tested as a ISTP at 18, 22, and 25 on the official MBTI. I am not and have never been one really... I just was scarred from emotional neglect by my parents, especially my mother, my father's avoidance, and my brother's emotional and verbal and relationship abuse. I was ready to be codependent.
I can see now how and why I closed myself off to pain. Consciously, I was a concrete thinker, an introverted problem solver. Unconsciously, I still acted as a ENFJ. Everything I did screamed ENFJ. My superego just hid it from me.
I don't believe I had an alternative personality, just imperfect thinking. I was able to do all the tasks of life pretty well, except I had a repressed sense of something being very wrong, and experienced a lot of depression and anxiety.
Similarly, my ISFP wife was scarred by childhood trauma. She tried to transform herself into a ESTJ, super competent and put together, but she fails at all the ESTJ things like planning, organizing, leading, delegating, etc. She still is trapped in that mindset, that ego construct, but she seems to be very slowly healing.
My mother in law, a ISFP, disassociated during her abusive childhood and trauma. She fights urges to kill herself to this day She acted like a bad version of a ESTJ until she had her hysterectomy and got on meds at midlife. She never did therapy and so still isn't healthy, but she is functional.
She has talked about her days as a young mother, angry, nasty, and controlling. She says she felt so out of control and lost that she needed to grab hold of control in order to function.
Trauma, abuse, and neglect hurt people until then heal. Healing, from my experience is very hard..... I hope to be done one day.....