Leaving at daycare

My 3 year old step son, doesn't really remember or knew his biological mother since she stopped coming around after he was 1 years old, she never asks about or to see him.

He's called me mommy since he was about 2 and half years old out of his own choice.

He is very independent, self soothed since was a baby, likes to do things in his own and on his terms.

He loves playing at his grandparents houses that he doesn't like to leave most of the time.

But now, he hates it when I leave him at daycare, he's been in the preschoolers room since mid July and now he acts out during the day and cries when I have to leave (when I started coming around the 2nd time he would cry when I had to leave and when I did move in, spent 4 months together until I got a job)

A few times when his dad has dropped him off, he's perfectly fine, but with me, he's developing attachment issues or something like that. I'm at a loss, idk if he knows there is something, someone missing from his life like his biological mother..