Won't fix:
Developers responsible for the area decided not to fix this issue, because addding safeguard preventing from unloading assets created/owned by the editor/engine is quite non-trivial in this case and there is a proper workaround of clearing the references of resources and using UnloadUnusedAssets instead.

To reproduce:
1. Create new project
2. Create c# script with following content:

Object[] textures = Resources.FindObjectsOfTypeAll(typeof(Texture));
for(int i = 0; i < textures.Length; i++)
{
if(textures[i].name == string.Empty)
{
continue;
}

if(textures[i].name != "LOADING")
{
Debug.Log("UnloadAsset: textures[i].name = "+textures[i].name);
Resources.UnloadAsset( textures[i] );
}

3. Attach it to camera and run