I thought the same thing, which sent me down a bit of an unexpected rabbit hole in the topic. Greg Paul argued that thr chevron shape of the bones in the bottom of the tail point to sauropods rearing and using their tails as support . Heinrich Mallison did some biomechanical modeling and found that some of the anatomical features previously thought to support rearing might actually hinder it. And last year, a study on larger sauropods (Dreadnoughtus and Giraffatitan) showed that their femurs most likely couldn’t handle sustained stress of resting.

So it looks like this pose is based on anatomy, not biomechanics, and the one rigorous biomechanical sauropod-rearing study that exists didn’t even test this genus - which means the rearing question Mamenchisaurus is unresolved.

https://en.wikipedia.org/wiki/Mamenchisaurus

https://reptilis.net/DML/2009Apr/msg00036.html

https://onlinelibrary.wiley.com/doi/10.1111/pala.70019

ALSO, consider how stiff their neck was, it could very well have spent most of the time grazing on the ground, like you said!