Hi ! I think that's an error.
It happened to me once, though by playing you'll notice that the nickname he is supposed to give you depends on the action you took when meeting him : like if you have chosen to flirt openly, he'd call you "dollface", but if you have chosen to be bashful, he'd opt for "sweetheart" instead. I don't quite recall how I happened to get the "sansfirstnickname" thingy though.
Hope it helped ? Even though you must have figured it out by now with how late the answer I gave is. 🫠
(Ps: Sorry if there are some grammatical mistakes)