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.
I don’t see this as an error. Yoda says as Luke is leaving Dagobah the first time - “Yes, yes to Obi-Wan you listen” - when they’re trying to warn him to not go to his friends. Anakin/Vader also calls him Obi-Wan to Luke. Luke naturally picked up on considering him as Obi-Wan instead of Ben Kenobi, his alas, from exile. He does though still call him Ben when they’re sitting together and having their more personal heart to heart. “I can’t do it, Ben.” I can count only three times when he refers to him by the name Obi-Wan. Once when he isn’t sure if who he knows as Ben is one of the same as Obi-Wan, with Leia on the Death Star, and the last time you talk about is when he says “Obi-Wan” before thet instance where they sit down and talk things out. You can see his slight irritation before they do. It makes sense within each context. Plus keep in mind Luke is only twenty three in Return of the Jedi.