I usually pick up the Unholy Reaver on the bodies of the Githyanki on L4 Watchers Keep as well.
I've done this on several installs of BG2, so I'd go as far as to say that it is usual to get it there. As Six pointed out, it does require a mod to be installed (one that I now install as default).
I would suggest that you have installed your mods/patches in an unusual order, and that the game is slightly borked as a result.
*cough* reinstall *cough*
