There is no bug.
I just respect my Demon Warlock today, gave him 20 hard points in Bind Demon, maxed both passives, maxed Bind Chance too and devided the rest as need, then went into Hell and got me a new Hephastos. I got 2 actually back to back since the first was not that great.
Therefore: Bind Demon works just fine, nothing to fix here. All you need to remember is that you NEED A FREE MINION SLOT and HOLD THE KEY DOWN, otherwise Bind Demon will not work!