Mecanim transition condition can't have both bool and trigger?

I'm developing a boxing game and when the AI is southpaw I mirror all animations.

I want the mecanim trigger BlockLeft to trigger either a BlockLeft state or a mirrored BlockRight state (when southpaw). I'm doing this by having two anim states. BlockLeft and BlockRight (with mirror set to true) and two transitions.

One transition to BlockLeft with conditions BlockLeft (trigger) and southpaw = false. One transition to BlockRight with conditions BlockLeft (trigger) and southpaw = true.

However, the transition to BlockLeft is always triggered, no matter what southpaw is set to. Is it not supported to have a trigger condition and a bool condition on the same transition? This seems like another bug in mecanim. I'm using 5.4.1f1.

Is mecanim generally buggy? This is the third issue I've ran into in a couple of days.