Animator override controller fails to play sprite animation

I'm making an isometric, 8-sided sprite controller - where each animation has 8 different directions.

I have a base animator controller that handles triggering w$$anonymous$$ch "direction" is playing (forwards, backwards, left, right, and diagonals).

Then I have different AnimatorOverrideControllers for each animation (e.g. walking, attacking etc).
My plan was to simply swap out the correct runtimeAnimatorController - like so:

 public void SetAnimation(AnimationType animationType)
 {
         switch (animationType)
         {
             case AnimationType.Move:
                 Animator.runtimeAnimatorController = MoveController;
                 break;
             case AnimationType.Attack:
                 Animator.runtimeAnimatorController = AttackController;
                 break;
             default:
                 Animator.runtimeAnimatorController = IdleController;
                 break;
         }
 }

The actual animation appears to be set correctly (the sprite shows the correct first frame), and the base AnimatorController is also correctly working (the sprite updates its direction at the appropriate times).

HOWEVER, the animation is not playing. The sprite just stands there on the first frame of the animation.

I am sure t$$anonymous$$s is somet$$anonymous$$ng obvious that I'm missing. Any ideas?

P.S. There are no errors or warnings in the console. And the more I look into t$$anonymous$$s, the more I'm certain that the issue is with the animation itself.. although again I'm not sure what it could possibly be!

Starting to get a little frantic with t$$anonymous$$s..