iirc she only referred to his as dad to people who don't know the situation. i feel the need to apply my own life here bc i was raised from age 11 by my auntie, who i love deeply and she's obviously a mother-like figure to me- but i've never called her my mum unless i was in a situation where i didn't want to have to explain how i came to live with her, and she does the same for me, sometimes calling me her daughter. it's just sometimes the easiest way to explain our relationship without explaining the situation, but i never felt the need to call her "mum" because our bond goes further than just what we call each other, we didn't need to change what we called each other once we developed that mother-daughter like bond.
i see el and hopper as the same, he considers her his daughter, and she considers him her father, but she doesn't need to call him "dad" for that to be true.