It kinda makes sense within cultural context i guess haha. Something similar happened to me too coz i ended up dating a family friend for a bit, and he was 36 when I was 25. As a filipino, I had grown up calling him Kuya (equiv of older brother) kinda like Oniichan/oppa/gege . So it was just natural for us to keep that when we suddenly started dating, and until now (back to being just friends) at 36/47 yo hahaha. But i did try referring to him by name when in a professional setting (his office) because he was a too lawyer at the time while i was a senior college student. Hella weird tho!! Coz in filipino culture it just feels so rude hahaha
i know koreans talk suoer formally but cmon youre in a relationship call him w his name or something else other than MISTER