Bass is usually mixed in the middle as well.
Not sure they're doing that on Atmos tracks though - I suspect they're doing it on the sub channel and then putting anything above 120hz in the front left and right - it won't make sense to have the bass panned around when you turn your head so i'm pretty sure the mixing guidelines on these will be to use the center channel for lead vocals. Don't get mixing on the centre channel confused with mixing bass in mono on a stereo track.