But here is where and why ROTJ is the oddball. Luke has faced Vader and lived. He starts out the story by rescuing Han. Jabba and his main henchmen and hangers on are all dead. Luke has mastered the powers and skills that Yoda taught. Luke is at the top of his game. He is the last Jedi (save Yoda on Dagobah). He turns himself in to Vader rather than risk his friends. He starts of determined not to fight. Why does he fight? The Emperor taps into his failings. The ones we’ve spent two movies dealing with. He incites Luke to draw and fight. That and Vader doing the same thing a little while later prove that Luke is not the poised Jedi Knight he is pretending to be. ROTJ shows that Luke is not perfect, but the overall impression is that he is now a full Jedi Knight. But he has a lot to learn yet. But now he has no teacher. The cracks showed on the Death Star. So when years later he establishes a Jedi school and half his students turn and kill the other half, Luke’s wave of success crumbles and his failings show for all to see. Just because they are nicely hidden in ROTJ does not mean they are not there. Many fans have speculated that he almost turned to the dark side - that he actually drew on the dark side to defeat his father. He has his father at his mercy and he hears the Emperor urge him to finish him and listens to something else that makes him stop and throw down his light saber (something he evidently did in a more permanent way after the fall of his school).
But to say that ROTJ clearly shows a changed Luke ignores all the flaws that the movie shows are clearly still there. The Luke of ROTJ is riding a wave of success and has matured, but those old tendencies to defeatism and recklessness are still there. Rian tapped into all the complexities of Luke from the OT when crafting the story for TLJ. If you gloss over the flaws that ROTJ shows are still there, or just ignore those parts of the movie, you get Luke the epic Hero. Luke the Legendary Jedi Knight. When you dig you find Luke the person and that person would do exactly what JJ and Rian have said he did. Even Mark Hamill has fallen for Luke the Hero. But once he got past the surface and realized what the story did, he agreed with it.
ROTJ is the aberration in Luke’s journey because it was his greatest success. His flaws were under control.