Honestly if it’s bad writing in ROTS it’s bad writing in ROTJ as well. They’re both retcons. Leia was never adopted until Lucas decided to make her Luke’s sister.

Sure ROTJ doesn’t necessarily directly contradict anything in the previous films, but, and maybe it’s just me, there still seems to be a logical leap in Luke and Leia’s conversation. Leia already knew she was adopted (and Luke for that matter)? Seems to come out of nowhere and never fit right for me.

That scene’s actually problematic in a few different ways. “Somehow I’ve always known” speaks for itself. We never really see Leia properly react to the terrible truth that Vader is actually her father (not quite enough emotion). It plays out much better in the novel (and probably also the script, though I haven’t read it), for example this final line: "“I have to try, Leia. He’s our father”