My DS is 2.5, he sees my parents for a sleepover once a week and since he's been talking better he calls them mummy and daddy. He calls us mummy and daddy too.
I think maybe he sees adults who care for him as mummy or daddy that's why?
We always correct him "no it's grandad" etc . Just last week he started saying grandma. I said we are going to Grandma's he said "grandma and daddy" I said no grandma and grandad.
Then when he came home, he called me grandma 😂 he's a confused boy!
I guess it's normal and he'll figure it out eventually?