Nick Viall and Vanessa Grimaldi are supposed to be engaged ... but you'd never guess it by the way they hang together when cameras aren't rolling.

Paps got America's newest favorite TV couple Thursday in Hollywood fresh off their 'Bachelor' finale earlier this week -- and they looked ... well, distant.

It's interesting ... they appeared cheerful as hell in front of a camera Tuesday in L.A.

But let's not forget their awkward onscreen reunion, which got people wondering if their post-'Bachelor' love was just a big sham.