I think it's their personal preference as to how they call each other.
They haven't yet reached the level in their relationship where they address the other with their first name. She refrains from calling him Rishabh just as he prefers Mrs. Bajaj to Prerna but he did call her that on several occasions.
First names are personal according to me. I don't think there's a twisted motive behind a simple title, it's a choice.