You cannot tell what Haki someone has until they use it.
Garp was surprised Luffy used Conqueror's Haki meaning he didn't know he had it.
Blackbeard met Luffy in Impel Down saying Luffy's Haki had become stronger... meaning he sensed Luffy's Haki back in Mock town.
It's possible to sense people have Haki energy, it's however impossible to sense or know what Haki they're capable of using.
Usopp can only use Observation, anyone can sense his Haki, they just can't tell what Haki he can use. Some people can only use 1 type of Haki, some can use 2 forms, and some can use all 3.
It's impossible to determine what Haki someone can use until they use it, it's something you determine from observing.
Mihawk couldn't know if Zoro could use King's Haki, he can only tell he can train and unlock Haki.
Being clairevoyant only implies you can see the future to some extent, it doesn't know you can know someone's entire future.
All Mihawk knows is Zoro is ambitious, has a lot of ambition, and thinks he has the potential to become a great Swordsman. Potential insight and knowing aren't the same thing.
There's nothing weird happening, they're simply not omniscient. You simply cannot know what Haki someone can use whether it's Observation, Armament or King's Haki until they use it.
Don't forget even Law had to ask who within the SHs could use Haki meaning he didn't know by default, he had to be told. And he was already a Haki user.
Pretty sure even Zoro was surprised when he saw Luffy use CoC back in Fishman Island, and Sentomaru was surprised when Luffy first used Armament after the time-skip. It's impossible to know what Haki abilities someone can use, until they use it.
At the very best, you can make an educated guess, you just can't know until seeing it.