Finally! i figure out the problem. First, it only cause on Editor. When Editor lose focus an get focus again, the unity "totalAllocateMemory" suddenly became lower. Then we load AssetBundle and Instantiate an gameobject which have animator on it.And we get the warning:Animator is not playing an animation controller. Animations is missing!
The way to solve this proble: Editor->Preferences->General->Script Changes While Playing set to "Recompile After Finished Playing"

And this error wont cause in 2019.1.10f1. And it is very easy to reproduce when i unload assetbundle and reload assetbundle.

I have the same issue. I'm on 2019.4.5f1. Like ILYES-GARIFULLIN say:sometimes everything works fine, and sometimes when calling Instantiate(), all the prefabs from the AssetBundle cause this error.

I think I found a workaround:
Every object I'm instantiating has a script attached to itself that copies the AnimationController from said object and then re-attaches it with a slight delay.
This seems to have fixed it.
Example implementation:

private RuntimeAnimatorController _animatorController;

void Start() {
_animatorController = GetComponent<Animator>().runtimeAnimatorController;
GetComponent<Animator>().runtimeAnimatorController = null;
StartCoroutine(_reloadStoryObjects());
}

private IEnumerator _reloadStoryObjects()
{
yield return new WaitForSeconds(0.2f);
_animatorControllers.runtimeAnimatorController = _animatorController;
}

Similiar problem here. Seems like the bug was fixed for a while and now people are stumbling upon it again. I'm on 2019.3.0f6

Same problem here and like Ilyes-Garifullin said it does not appear every time. Very strange.

I have a workaround that "seems" to work.

Instead of just simply setting the runtimeoverride controller, I create a new one and copy the animation pair data over.

public static void DeepCloneAnimatorOverride(Animator animator, RuntimeAnimatorController originalOverrideController) {

AnimatorOverrideController newAnimatorOverride = new AnimatorOverrideController(originalOverrideController);

List<KeyValuePair<AnimationClip, AnimationClip>> pairs = new List<KeyValuePair<AnimationClip, AnimationClip>>();

for (int i = 0; i < newAnimatorOverride.animationClips.Length; ++i) {
pairs.Add(new KeyValuePair<AnimationClip, AnimationClip>(newAnimatorOverride.animationClips[i], originalOverrideController.animationClips[i]));
}

newAnimatorOverride.ApplyOverrides(pairs);

animator.runtimeAnimatorController = newAnimatorOverride;
}

And the most interesting thing about this bug is that it doesn't appear every time the game starts, sometimes everything works fine, and sometimes when calling Instantiate(), all the prefabs from the AssetBundle cause this error.

I have the same issue, and I'm not playing the animation before my model is instantiated. This warning throws when I'm instantiating my prefab from assetbundle.

You are playing the animation before your model is instantiated. I had this issue before, try moving your Animator trigger after instantiate of your model.