Nursing bovine adults often leave newborn calves by themselves, curled in a ball, for hours at a time. Very normal as the calve risks injury early on if it tries to stay with other adult animals in the herd.
Not a sign of rejection at all. A rejected calf is obviously sick or deformed, even to the relatively untrained eye.
Leave wild, baby animals alone, they need no help.
While I can’t disagree with your last line, and am no expert on Buffalo, I do kno that sheep will reject for no discernible reason. Unless one owns th animal, I would advise not getting in the way, especially if the animal is a buffalo.