It is consistent. Just pay attention to the manga itself and not fanfic writers on forums and youtube.
You see Hardening because it's a specific move that makes haki visible while physically hardening the body part/item its used on. Dressrosa clarifies this with Mihawk's "every blade can be made black with haki" dialogue as well as the debut of Gear 4 when Luffy using full-body Hardening
physically changes the properties of his rubber which lets him do different compression-based techniques that he cannot use without Hardening.
People with the ability to use logic can read these 4 consecutive pages from Punk Hazard and realize that when Luffy draws blood from full-Logia Caesar and Caesar says "that hurt!" he's using Haki, and that invisible haki is a different technique compared to Hardening. You can even see Caesar
look at the black coating with his eyes and panic.
People with water in place of a brain read these 4 consecutive pages and go "uhhhhhh i guess Oda forgot, because Hardening is just for the readers" (some shit Oda literally never said, entirely fan headcanon, complete bullshit that spreads from youtubers)