sigh. my guess :
kishi wants to show as many reactions as possible , yet panel spaces are limited and those reaction panels has to have similar patterns in order that the manga layout looks natural
since hinata happens to be not as near as kiba's dog and shino , therefore that panel cannot encomposs that much range to contain her as part of the comic relief . hinata wasn't shown to have reaction =/= she didn't have any reaction .
to me , hinata tends to react more calmly than shino does in the usual days . maybe she's slow to react to this kind of astonishment , or that a blank facial expression is her way to react to shock news (unless it's sth that would embarrass her)