There definitely is a glitch here! Yesterday I tried to book it as a return flight AUH-LAX-AUH and there it was! Wide open availability of all the flights using miles!!
I called up Etihad call center and booked it as a return without issuing the tickets and the staff had to manually remove the outbound in order to give me the return sector of LAX-AUH.
Seems really weird but eventually I got it!