I was just in act II doing bounties in the oasis and came across the keywarden, I took him down and he dropped infernal machine plans. Doesnt the Act IV keywarden drop the plans and this guy drops a key??
If this is old news let me know, while not specifically key farming a key would have been welcomed, not the plans.
Keywardens actually also have a chance to drop a different key. I got the Key of Bones off the act 2 warden twice in a row and thought for sure it was bugged, till I looked it up and saw they had a small chance to drop a different key. Sure enough, I ran him again and got his normal key (Gluttony).
Also, if you learn the plans, I believe you should never have them drop again. (Both keys and plans are completely separate from the ones at level 60, if that is the source of the confusion).