it actually happens to me as well.
i can actually still manually force him to attack, but as son as the mob dies he goes back to do nothing.and i canòt even dismiss it
however it does not happen all the times, and sometimes if i die or get teleported the next minion i summon wonàt be bugged and i am able to set him to assist or do whatever as normal.
i am pretty convinced it's some kind of bug, just maybe not everybody is experiencing it yet.