I'm having this exact same issue. When i stand still, my check says "false", and when i start moving the check flickers back and forth between true and false. I thought maybe it had something to do with the fact that my character was colliding with a mesh collider, but i tried a ground made of a simple cube and got the same results.
I'm happy to post up some screenshots if necessary. Has anyone else run into this and been able to fix it?