The thing with Haki is that the more extreme one's battles are, the stronger their Haki gets in the process, the more it evolves. I don't recall it being really that inconsistent. It just reached a level where the immense gap that was present beforehand closed the longer the fight went on. It can feel kinda weird at times but from my perspective it was purposefully given that growth mechanic as a comfortable "excuse" for instances like Luffy's Advanced CoA phasing through Kaido's CoC coating, albeit, doing no significant damage in the process. Later on Luffy managed to actually clash with him for a short period once he figured out how to utilize CoC coating and then managed to land more significant blows on him, albeit, to no avail because he ended up getting knocked out without doing much to Kaido aside from being able to match him 1vs1 for a short period.