Definitely feel this with 5e. I've never ever, ever, ever seen a hexblade use the fluff from the book. It's always an old one hexblade, or a celestial hexblade, or a mechanical hexblade, or a guy with an intelligent weapon. If I hadn't read the book, I never would have known that the default hexblade relates to the Shadowfell.
I feel like one might make sense of the notion of dissociation by thinking through the implications of your and
@Mordhau's comments. You can see that it would most meaningfully matter to be volcano barbarian or hexblade warlock if there were mechanics that connected to the driving in-world fictions. That would be narratable, and not easily covered by refluffing.
For example, if volcano-barb gains Con when standing on a volcano, then crunch and fluff are chained together. (I'm not saying that would be a great subclass feature! It's just to illustrate the idea.) So 'dissociative' might be meaningfully redefined as lacking valency to the fiction that it's
intended to have valency to. In the case of 4th edition, the 'problem' is that for much of the audience, the intended fiction is taken to be centred on European-mythic-medieval-fantasy-light - bearskin-clad barbarians, bookish wizards, scaley dragons, and all that - as remolded through the cycles of D&D IP development. 4th edition mechanics were superficially, but not deeply chained to that expected genre. They were deeply chained to a quite different genre.
That produces jarring issues like that
@UngeheuerLich encountered. A lack of language to narrate what the mechanics were doing. I think if you read back over some of the (quite lengthy) discourses on this subject, such as
@Lyxen's, you can see this as the underlying problem. Many posters frame it as an objective problem, when it is principally a
subjective problem: a problem of what has been normalised for them. The objective aspect is
limited to the problem of what they might reasonably expect, based on what the game designers have said they intended (including via the game-as-product positioning).