This is not a bug, but it is a problem
A quick explanation of the problem Phobos had (he's since overcome it), and which is genuinely confusing.
If you look at his screen shot you see the field Criteria is at 0.0%, but how do you raise it?
After the pet reaches the required level, spawn the pet and tame other mobs while your pet is watching. Each successful taming will progress the Criteria %.
When Criteria reaches 100% (and all other conditions are met) the Passive Effect can be unlocked.
Once unlocked, the Passive Effect can be turned on and off as needed.
Cheers, Miles
P.S. Gratz to Phobos on the big Yashib find!