Was playing MOP intro yesterday, got up to Old Man Liang quest, logged off and I log in today and hes gone. I walk down the hill from his place and he reappears, I see him and can click on him from all the way bottom, however when I start making it up the hill and zone changes from pools to liangs retreat he disappears.
Update* If you are having this issue as well, thanks to wowhead I found out that he has another location just a bit below from his original place where he appears. 78, 43