I feel people hold Zoro higher than any other member of the straw hats. Oda likes to test SH faith/trust directly.. even after the timeskip. The thing with Sanji in WCI is the same thing. Sanji didn't have enough trust and faith in Luffy

But Zoro is immune? #onepiece

It's all about building the story around the straw hats insecurities. Nami in arlong park had learned to bear the burden the test was to teach her to ask for help.

Sanji is similar. Sanji needed to learn to place himself as high as others - he is vital.. ask for help

If Zoro has an insecurity that Oda can use against him to test him... I believe it will have something to do with defeat

His flaw may be pride