I don't think this is anything new at all. It's the main reason I gave up live-watching Dramas several years ago. K Dramas have ALWAYS had a history of botched landings, and after a few years of putting up with (2014-2018) I gave up and only start shows after they've aired. Strictly from my limited experience and in personal opinion, possible reasons include:
A change of writers is a common reason - it happened A LOT back in the day. Other factors that are less common now than formerly (but still do happen) are when shows have their episode count changed - either extended or cut, according to ratings. That destroys works without fail (King2Hearts!) and is often not the writer's call.
Genre-mixed shows might also be part of the problem. For example, basically every K Drama "romance" now has to have a serial killer/psychopath included, and often Dramas with very different genre elements may be written by a writer skilled in only one of those genres. Or, the Drama is a collaboration and the different genre contributors don't gel. And finally, there's the issue of series length. I'm hoping that this may become less of an issue as shorter K Dramas become the norm, because Dramas that depend on pointless filler to pad out runtime often crash and burn in the finale
