Non-multisampled texture being bound to a multisampled sampler error

Hi to all,
After I upgraded my project from Unity 2018.1.7 to unity 2018.3 I encountered an error and I cannot resolve it.
My project downloads on demand some assetbundles (3D textured objects), so I exported these assetbundles with the new version of Unity.
When I run my app built with the older version of Unity, and I download the assetbundles (exported with the new version) the editor says:

"A non-multisampled texture being bound to a multisampled sampler. Disabling in order to avoid undefined behavior. Please enable the bindMS flag on the texture"

"Multisampled textures not supported for the requested texture type"

"Default texture must have a valid texture (texture dimension 0)"

I don't know if this information is useful, but the materials I put in the assetbundles are in most cases Standard Materials, with Normal Map and an Albedo Secondary Map. The Albedo Primary Map is stored inside my app (offline) and is added after the assetbundle is Instantiated.

I noticed that the problem appear only on iOS metal devices

I don't understand these messages, and I'm not expert in shaders. I woluld like to make the new assetbundles compatible with the old version of my app if possible. Could you help me?