When building atlases in PNG instead of Unity textures, I've realized that they are not unloaded between each application's start.
The RAM get filled at each press of play, eventually leading to Unity's own "Blue screen of death".
This error does not occur when building atlases in Unity Texture.
I've noticed this leak while outputting materials name and InstanceId from Resources.FindObjectsOfTypeAll(typeof(Material)).
I hope it will help you to locate the bug.