I tested this with Magram centaurs (at southeastern Desolace). Everything works fine, until i'm unfriendly with them. Then i have to constantly uncheck and recheck "at war" in the reputation window in between almost every fight, because if i don't do that i receive the "invalid target" message when i try to attack them. This is the only way to kill them.
update: after moving from unfriendly to hostile, this doesn't happen anymore. The bug only happens when i'm unfriendly with them.
(tested on covenant-wow official mangos server)