When I add a model with textures that include transparency (like a fence or decals) into a scene, the transparency doesn't work correctly. After some trial and error and research, I found a solution. You need to open the material instance and change the parent texture to the correct one.

Here's how to do it:

This method effectively resolves the transparency issue. I'm sharing this in hopes that the issue can be fixed in future updates, allowing models to be drag-and-drop ready. For glass, I believe using the Master_transparent or Master_translucency materials should fix the problem, although I haven't tested this yet.