Animator stops working after updating to 5.6

Hello. In Select Player scene of my project, I change characters by loading runtime animator controller from Resources (code below). It worked fine in 5.5.

 void UpdateSelectPlayer () {
         playerAnimator.runtimeAnimatorController = Resources.Load<AnimatorOverrideController> ("PlayerSelectPlayerAnimators/" + firstLetter + secondLetter + thirdLetter);
         mapAnimator.runtimeAnimatorController = Resources.Load<AnimatorOverrideController> ("MapAnimators/" + forthLetter);
     }

But after updating to 5.6, animator doesn't work anymore. The problem is when I select a new character, the sprites showing on screen is still of the old character and animation freezes. However I see in the Inspector that animator controller changed to the animator override controller of the new character I select correctly. And when I select the old character, animator works (animation happens) again.

I already checked by selecting a random character when the scene started by script. Animator of the first selected character always works no matter which character it randomly selects. The problem happens again when I select others that's not first selected when starting the scene.

I'm really not sure what happens. This select character is quite a big features for me and I spent a long time implementing. Thanks for any help.