Return of the Jedi very much ends with “And they lived happily ever after”, so seeing the Luke that rejected the dark side and turned Vader to the light fall prey to the very doubts and fears that he successfully resisted may seem counter to expectations. However, real life is full of regressions, relapses and setbacks. People contain multitudes, and personally I prefer my characters to be messy.
I think that the thing with Luke is that we don’t see a natural fall of the character. The last time we see Luke in ROTJ he is happy and well-balanced, and suddenly he is completely different in TLJ, as Octorox said. However, the problem is that the reason behind his fall is just showed in a brief flashback and explained in a really superficial way, and this is why it was received so bad by a big part of the fandom.
Kenobi, for the other side, has a fall super acceptable, since we saw in ROTS all the motives for him to being that way in the show.