Board Thread:Frustrations and Venting/@comment-4449097-20140717225903/@comment-16147032-20140718085417

Yes the rewards change from boss to boss. The 2 bosses released on day 1 (in this case Tankor and Sky Bite) both give out free ECs. No other boss after day 1 will give back EC. If you did actually face this bug it sounds very unique, I have not heard it reported anywhere else.