My D’s ex called us by our first names, unasked. That was fine. I can’t think if I’ve ever heard my S’s long time gf call us anything at all, haha.
My kids rarely call me anything unless they want my attention, haha. If I’m not hearing what they need to say, I get “Linda!” It is a bit weird I guess.
I have a friend who I’ll call Dora. Dora’s kids have always called her Dora, or, these days, Daz. But the kids’ girlfriends/boyfriends refer to Dora as Mrs. Smith.
I used to nanny for a family and all the kids called the parents by their first names. Not the same as SO’s, but I think a lot of it has to do with what a person’s parents taught them growing up.