New information in the same ongoing story or linear line of events is almost always going to reframe or re-contextualize past events, though. It's almost impossible for it not to unless you constantly shift both characters and settings (in which your narrative arguably isn't linear anymore, or even the same narrative). What you believe is true of Lucas, for example, is just more subjective justification for whether or not you want to employ the term "retcon" and doesn't speak to either a flaw in composition or in construction that said retcon may or may not have caused. I'm saying it doesn't actually matter one way or the other, because the plasticity of narrative supports the contextualizing of the new information without breaking continuity in any real way, it's just new information dynamically shaping the way in which you digest the story.
To answer your question in regard to Frostmourne, the answer is "no," this expanded lore wasn't at all indeed from the beginning, but it's that very requirement that is at issue here. It's not required that the new lore have justification retroactively, it need only be able to peaceably coexist with the previous lore, which it can and does, at least insofar as this Frostmourne thing is concerned. Now there are some elements of Shadowlands that don't meet that criteria, unfortunately, and cause some issues with digestion. For example how Necromancy works with beings like Kyrian aspirants or Soulshaped Night Fae stewards, etc. I think the soul-splitting behavior of Frostmourne actually supplies a good deal more explanation for a few things, like why Sylvanas became the way she is/was, as well as giving Uther an expanded and interesting story despite his death early on in Warcraft's story. From the subjective standpoint, it kind of depends on what a given person wishes to accentuate or dwell upon.