It is also weird Luke referring to Ben as Obi-Wan for the first time in ROTJ. I wonder if that is a writing error as well, or is it something that happened production-wise. For the past 2 movies and 30 years of Luke’s life he’s always been Ben to him, and then a couple of years after his death and when he has seen Ben for only 2 times after that, he starts to call him Obi-Wan? It just doesn’t fit into the story.
It’s been a while since I watched the OT, but I’m willing to theorize that the events of Empire got Luke shaken up enough that he started to call him Obi-Wan instead of Ben, due to feeling somewhat betrayed. My memory is hazy for tiny details like this one, but if Luke calls him Obi-Wan during the Dagobah visit in ROTJ, it’s a possible idea.