Hi,
had the same problem, that the usual way like you did:
nohup <cmd> &
did stop running, when i logged off. We guess it's some kind of bug, because it's only happening on an AIX 5.2 machine we have installed. All our 4.3.3 behave fine. Don't ask me why this problem occurs, but there is a solution that works and let me log off while the process is still alive:
su -c <username> "nohup <cmd> &"
Try it; it worked for us.